Because they have three separate ranges of vision.I must wear progressive lens as my vision is to the point I can no longer see clearly anything at any distance from me,with out some sort of corrective lens.as you move your eyes up or down the lens changes strength .the trick is to learn how to use them and for what you are looking at. the same with tri focal lens except one only has 3 different lens to look thru.
After the age of 40, the lenses of out eyes become progressively less flexible and have trouble accommodating to focus on objects at different distances. When you're young, the lens is held in place by lens zonules. The lens zonules are strong and they are able to move back and forth to bring images into focus. As you grow older, these strings start to relax, which makes it more difficult for you to control the movement of the lens to bring an image into focus. As these lens zonules relax, you can either hold an object farther away or use bifocals to bring an image to a more comfortable position. Bifocals are the cure for arms that are not long enough.
As we age, many of us notice difficulty reading small items within a meter (or few feet) of our eyes. This issue can become more pronounced after the age of 40, when the lenses of the eyes become progressively less flexible and have trouble accommodating to focus on objects at different distances. Some people who are just noticing difficulty viewing near objects can continue without bifocal glasses for several years. It may, however, take a few seconds to adjust one's focus when looking at a faraway object and then at something close. For people with just occasional difficulty seeing close up, using a pocket-size magnifying glass for reading small text and then your usual single-vision glasses/contacts for everything else, such as driving or watching television. Still others use two pairs of glasses (one for far distance and one for near) and switch back and forth as needed. When quick fixes such as these become inconvenient or inadequate. there are many other refractive options to
consider,Progressive lenses make a smoother transition between as many as 20 different focal points in the glasses (for clearly viewing varying distances) compared with conventional bifocal lenses, which typically have a line dividing two parts (or trifocals, which have three parts) within the glasses. While wearing bifocal contact lenses in both eyes is a common choice.