The idea is to redistribute wealth from areas of the country with wealth to areas are poorer.

California tax payers support West Virginia infrastructure.

Inevitably some states are wealthier than others, the federal government acts as a balancer of this. This can improve outcomes in poorer states (education, building up new industries, local economies etc).

Ideally, it also provides a counter balance for changing economies, where there are inevitably winners and losers. Many industries are geographic centered, and having the ability to adjust for acute downsides in one area benefits the whole country overall in the long-term.