why should we keep the mesh on quads geometry ?

All-quad geometry is standard modeling procedure for non-game models. Quads smooth better as subdivision surfaces and they’re generally more conducive for deformation (rigging/animation).

I recommend all-quads as general rule, however sometimes I will let a triangle sneak in occasionally. But for ares of increased deformation like shoulders, elbows, knuckles, knees and hips, I highly recommend all-quads.

Some people freak out about triangles in a wireframe. I’m not one of those people, but I do see them as a last resort.