@umbra21 - In some cases people can't afford anything else, but I wouldn't use those cheap reading glasses if I had an alternative. A lot of people (myself included) have two different prescriptions, because their eyes are at different strengths, and it's possible to hurt your eyes by wearing the wrong prescription for too long.
I also think it's a good idea to go to the optometrist reasonably regularly because there are a lot of other things they check for. I didn't know I was nearsighted until I was checked, as the main thing I noticed was that I would get headaches at school for what seemed like no reason.
They also test for diseases that can show up in the eye.
I do find it too expensive to get my glasses from my optometrist, so I usually just go there for my prescription and check-up then get the actual glasses from online.