reason to have entire Cu layer as GND connected through via (KiCad)
Component mount:

There is no specific reason to mount the components at the top side. there are some board that double sided mount. However, it is conventional to use the top side for the Components mount.

GND layers:
Ground layer are useful especially in building Anlog mixed Board. 
any voltage can be written in V= (V+)-(V-),so the Ground here is the V-=0V. However if we limit the path of the returning current ( GND path ), we can have an induced voltage there like V-= 0.001V . so, For some components, this can be an issue.

so to overcome this issues we use the ground plane as all the components will always have the same V-.