Why do peopel need progressive lenses？

Invalid of single vision：
When people over 40 years old, one pair of single vision glasses might not be able to meet their requirements. They could see distance but not close, or could see close but not distance. At this time, they do need to wear two pairs of glasses, reading glasses when used to see close things and distance glasses to see distance. The other way is to wear multi-focal glasses, and the multi-focal glasses include bifocal and progressive glasses. The multi-focal glasses are one pair of glasses can be used to see distance and close up, you could use the upper farsighted part to see distance and the bottom part to see near things.