1. Bulking Agent – Non-nutritious or inactive substances added to increase stability of the mixture.
2. Emulsifier – Allows water and oils to remain mixed together to form an emulsion, such as in mayonnaise, ice cream, and homogenized milk
3. Flavor / Flavoring / Flavor Enhancer – Provides or enhances a particular taste or smell.
4. Gelling Agent / Thickener – Increases the viscosity by thickening the liquid to give it more texture
5. Humectant – Binds with water to increase skin hydration. Also enhances water absorption of the skin
6. Sweetener – Sugar substitutes which provides a sweet taste without raising blood sugar levels
In food and beverages, glycerol (E422) serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g., cookies), and as a thickening agent in liqueurs.
Glycerin/Glycerol is used in medical, pharmaceutical and personal care preparations, mainly as a means of improving smoothness, providing lubrication, and as a humectant. It is found in allergen immunotherapies, cough syrups, elixirs and expectorants, toothpaste, mouthwashes, skin care products, shaving cream, hair care products, soaps, and water-based personal lubricants.
Glycerin is a trihydroxyalcohol with localized osmotic diuretic and laxative effects. Glycerin elevates the blood plasma osmolality thereby extracting water from tissues into interstitial fluid and plasma. This agent also prevents water reabsorption in the proximal tubule in the kidney leading to an increase in water and sodium excretion and a reduction in blood volume. Administered rectally, glycerin exerts a hyperosmotic laxative effect by attracting water into the rectum, thereby relieving constipation. In addition, glycerin is used as a solvent, humectant and vehicle in various pharmaceutical preparations. 
Fun Facts :
-A mildly sweet, colorless, odorless, rather viscous liquid
-Glycerin can be packed into a pill and stuffed up the rectum. This draws water to the intestines to cause a bowel movement. It belongs to a class of hyperosmotic laxatives (absorbs water to make you poop).
-Can be mixed with nitric acid to form nitroglycerin, an explosive
Recent Findings :
Glycerin can be used to induce a wide variety of symptoms such as hearing loss,  hypotension  and hemolysis. 
Glycerin is also “an excellent preservative used commonly in immunotherapy extracts”.  Though it may be a skin irritant for some individuals , but can also be used to make the skin more supple and improve "skin mechanics" as a humectant.  
Nevertheless, glycerin extracted from the seed oil of the Jatropha plant may have toxic effects.  When heated for use in E-cigarettes, "vegetable glycerin can generate acrolein and glycidol, as well as formaldehyde".  However, long term studies have not been conducted on humans to determine the severity and scope of unwanted by-products.
In a 2 year study involving Long-Evans rats, they were fed a diet containing 5, 10, or 20% natural or synthetic glycerin. The study showed benign tumors in all glycerin groups but not the control. Despite this, there were variations in organ/body weight ratios, histopathological changes, albuminuria and glycosuria though all of them were "sporadic and did not demonstrate a clear dose dependency".  Although glycerin may be linked to an increase in tumor incidence, there is no correlation between physiological changes and the dosage of glycerin given. Glycerin can also be used as an alternative feed for poultry , where glycerin is a precursor to glyceraldehyde 3-phosphate, an intermediate in the glycolysis and gluconeogenesis pathways. 
Overall, the majority of glycerin-induced negative health conditions stem from the injection route of administration, and not oral. Glycerin consumed orally is rapidly metabolized in the human body via glycolysis/gluconeogenesis pathways.
Scientific References :
1. PubChem: https://pubchem.ncbi.nlm.nih.gov/compound/753
2. The Caloric Response in Menibre’s Disease During Spontaneous and Glycerin-Induced Changes of the Hearing Loss. (Acta Otolaryngol., 71(1-6), 462–468. doi:10.3109/00016487109125390)
3. Mechanisms of baroreceptor-induced changes in heart rate. (Am. J. Physiol., 218(1), 251–256. doi:10.1152/ajplegacy.19188.8.131.52)
4. Cyclodextrin-induced hemolysis and shape changes of human erythrocytes in vitro. (J. Pharmacobio-dyn., 5(9), 741–744. doi:10.1248/bpb1978.5.741)
5. The GILL study: Glycerin-induced local reactions in immunotherapy. (J. Allergy Clin. Immunol., 121(1), 222–226. doi:10.1016/j.jaci.2007.08.016)
6. The influence of water, glycerin, paraffin oil and ethanol on skin mechanics (Acta Derm. Venereol. 01 Dec 1993, 73(6):404-406 doi: 10.2340/0001555573404406)
7. The influence of a cream containing 20% glycerin and its vehicle on skin barrier properties. (Int. J. Cosmet. Sci., 23(2), 115–119. doi:10.1046/j.1467-2494.2001.00060.x)
8. Application of time-of-flight mass spectrometry for screening of crude glycerins for toxic phorbol ester contaminants. (J. Chromatogr. B., 1046, 226–234. doi:10.1016/j.jchromb.2017.01.005)
9. Cardiovascular effects of electronic cigarettes. (Nat. Rev. Cardiol., 14(8), 447–456. doi:10.1038/nrcardio.2017.36)
10. Comparative toxicity of synthetic and natural glycerin (AMA Arch Ind. Hyg. Occup. Med. 1953 Apr;7(4):282-91)
11. Toxicological studies on synthetic glycerin. (J. Am. Pharm. Assoc. (Scientific Ed.), 39(10), 583–585. doi:10.1002/jps.3030391014)
12. Glycerin-A New Energy Source for Poultry (Int. J. Poult. Sci. 2010, 9(1), 1-4. doi:10.3923/ijps.2010.1.4)
13. Prolonged Control of Increased Intracranial Pressure With Glycerin. (JAMA Neurol., 27(1), 95–96. doi:10.1001/archneur.1972.00490130097015)
14. Glycerin. (JAMA Ophthalmol., 70(5), 625. doi:10.1001/archopht.1963.00960050627008)
15. Gastrointestinal Irritant Effect of Glycerin as Compared with Sorbitol and Propylene Glycol in Rats and Dogs. (J. Pharm. Sci., 56(3), 398–400. doi:10.1002/jps.2600560321)
16. Glycerin, Ethylene Glycol, Propylene Glycol and Diethylene Glycol. (JAMA, 109(19), 1517. doi:10.1001/jama.1937.02780450021006)
17. Acute toxicity and laxative activity of Aloe ferox resin. (Rev. Bras. Farmacogn., 23(2), 279–283. doi:10.1590/s0102-695×2013005000009)
18. Oral Glycerin in Cataract Surgery. (JAMA Ophthalmol., 73(4), 516–518. doi:10.1001/archopht.1965.00970030518014)
19. Treatment with Glycerol of Cerebral Oedema due to Acute Cerebral Infarction. (Lancet, 298(7732), 993–997. doi:10.1016/s0140-6736(71)90321-7)
20. Effects of glycerol treatment and maintained depolarization on charge movement in skeletal muscle. (J. Physiol., 254(2), 285–316. doi:10.1113/jphysiol.1976.sp011233)
21. Feeding Glycerol to Transition Dairy Cows: Effects on Blood Metabolites and Lactation Performance. (J. Dairy Sci., 87(12), 4195–4206. doi:10.3168/jds.s0022-0302(04)73564-x)
22. Glyceride Hydrolysis and Glycerol Fermentation by Sheep Rumen Contents. (Microbiology, 25(2), 215–225. doi:10.1099/00221287-25-2-215)
Regulatory References :
1. US FDA Food Additives Status List 
2. EU Approved Food Additive 
3. US FDA Generally Recognized As Safe (GRAS) Food Substances