If you don't wear glasses with myopia, your prescription power will increase. Moreover, it may affect you in many ways. Your bad vision will cause strabismus or amblyopia if you don't wear glasses to help you maintain your eyesight.
If you don't wear glasses when you need, you may have blurry vision when seeing far away and even get a headache in the long time. If you are slightly nearsighted, you don't need wear them all the time and just use them for distance vision.