It evens out tire wear. Front and rear tires on a typical FWD car wear at very different weights. I am different from most people and don’t feel like with my RWD vehicle I should rotate the tires if they are wearing evenly. Rotating them can mask an alignment problem that might be picked up by observation of the bad tire wear.
Rotate them as often as it tell you in your owner manual service schedule or as often as the tire manufacturer says to if you want to preserve your tire wear warranty. That said, the latter is probably not worth the paper it is printed on anyway. There are too many loopholes. Rotate them as directed in the owner manual.