Reading glasses are made primarily for presbyopia which is an age-related eye disease most people experience around 35 to 45 years of age. Presbyopia is caused by a decreased elasticity in the eye's lens, making it more difficult to focus on objects in a close range.
Reading glasses come in generic strengths that can be picked out by a consumer via a trial and error process. While reading glasses can help correct problems related to close-range vision, they are not made to correct long-range vision problems. Furthermore, for people who are experiencing presbyopia, wearing reading glasses may hinder their ability to see long-range. For this purpose, bifocal reading glasses were invented, which use two lens powers for distance and close range vision.
Finally, reading glasses are typically made with lower quality materials than prescription eyeglasses, and they won't give the wearer the accurate vision correction a pair of prescription glasses can give.