I have a degree in Mathematics and I wondered the exact same thing. My professors explained to me that we are actually taught math backwards. Like we learn that 2 + 2 =4, or the square root of 2 is irrational, etc. when we are very young. Most people really never understand the reasoning behind this, but there is a reason why. The explanations also known as proofs, theorems, corollaries, etc. prove these things as true and are taught in upper level mathematics @ the college level. Another example,Calculus,is really advanced algebra, that builds upon the simple principles you learn in grade school. People use this math for engineering, movement/change, etc. there is even a formulation for the undulations of tides (believe it or not). In general this math (Calculus) we use to solve certain difficult problems, all based on simple principles, just expanded. So advanced math certainly has its place, but most people never have a use for it.
As a side note, with my degree I went all the way to Advanced Calculus 2, and right now I can't tell you half the "crap" I learned. The basics, however, addition, subtraction, multiplication, etc. are still firmly embedded in my brain. I guess if you don't use it, you most certainly will forget it.