Actually I think the ready-made reading glasses can do harm to your eyes. The reasons I can give below.
Because you do not need a doctor's prescription, many people just eyeball their prescription when it comes to reading glasses.
Vision doesn't just vary from person to person, but also from eye to eye.
Many people have different prescription in each eye, and wearing glasses that don't cater to each eye can cause unnecessary strain and discomfort.
Ready-made glasses don't address unique things like astigmatisms.
Because they are not made specifically for your vision, you may experience headaches, nausea, and tired eyes, especially if you wear ready-made eyeglasses for a long period of time.
Also, it should be noted that reading glasses are different from computer glasses. Because reading is done much closer to your face than reading a computer screen, you may experience strain trying to use one in place of the other.