Because sidewalks are usually in places where the sun can shine
on them most of the day. Even if the sun can't shine directly on them,
the weather can get hot. Either way, the material that the sidewalk is
made of can often get hot. You know that when things heat up they
"expand" ... they actually get slightly bigger. If there were no spaces
in the sidewalk for it to get bigger, then when the material got hot and
tried to get bigger, it would crack and break, and the whole sidewalk
would be all broken up in a few months.
