The second website I came across is written by   Guillermo Bautista .  He talks about how painters use perspective to create depth, and to show if something is far away, or up close. Painters use variations of perspective such as two-point perspective, three-point perspective, and atmospheric perspective. When you are drawing to create depth, it involves ratios of the different lines and shapes. 

Painters such as  Vincent Van Gogh used perspective in his painting Flower Beds in Holland use these techniques to make it look like a photograph.

The Mathematics of Perspective Drawing