Why are so many studies done on rats? I don’t understand how they relate to humans.

Closely related. Many times, early clinical trials are done to figure out how a drug or treatment will work, etc. There are well known "models" of various diseases that are in rats, and so, many experiments which would be harmful if done on humans are usually first done on rats. Whenever a treatment shows promise, however, there are always human trials, with very strict safety guidelines.