As we age, our bodies produce less and lower-quality collagen. Typically this results in wrinkles, sagging skin, and sore joints. Supplementing with collagen can relieve joint pain & improve skin elasticity.
There aren't many known side effects from collagen, but like all supplements, you should research and talk to your doctor before using.
Collagen is essential to maintaining good health, so people often supplement to ensure their body has enough.