Question

Why the ATOM have shells.?

Answer 1

Electrons are tiny subatomic particles with a negative charge that orbit in shells .so the atom have shells.

Answer 2

Atoms technically have orbitals and not shells but they  have simplified it as shells 

so shells are imaginary pathways in which electrons revolve around the nucleus.

there shells because of the force of attraction of the nucleus over the electrons each electron experiences a different amount of force so they revolve around in different distances.