Question

why is it necessary to have spaced gaps in the sidewalk

Answer 1

Its called control joints the slab of concrete will crack after a while so this will help prevent that

Answer 2

-- Practically all substances expand when they're heated.
Concrete is no exception.

-- The sun beats down on the sidewalk most of the daytime and heats it,
so the sidewalk material expands during the daytime.

-- If a material is confined, pressed tight against something else,
but needs to expand, it either buckles or else it cracks.

-- We don't want our sidewalks to buckle or crack when they get warm,
so we have to give them some space to expand into.