Different proteins in the mash need to do their own thing (biochemical reactions); however the optimal temperature for any one particular protein to do its thing is different. So you start low ~100 F and let those proteins work, then you crank up to ~115-125 F and let those proteins work...all the way to 145-160 where you let other proteins work (amylases).
Once you get really hot >180 F - proteins start to denature or unravel and are unable to perform their functions anymore.