Why do members here buy seat covers?

Depends, some people lead a "dirty" life and all kinds of crap gets into their truck. Definitely a good cover candidate. Others want to keep the new seats looking new. Others, like me, like to use the stock seats until they get to a certain level of worn, then consider covers.

Also keep in mind that if you rat out the seats, even if you have covers on, the resale value will be affected. Don't know about anyone else, but when I buy used, if it has covers, I check out what's going on underneath and value it accordingly.