The gpu is graphics processing unit,it takes care of all the graphical calculation and algorithms for graphics
renders textures,select apt resolution etc
for a computer that does not have gpu the CPU has to do all the work,graphical + all the other functions (so its slower game-play without a GPU)
1) it takes all the graphic job
2) yes you can call it as an assistant
(gpu does the job faster and effectively for it.because its built just for that)