What is the purpose of having three levels in the federal court system?

The purpose for having three levels in the federal court system is to keep different crimes of issues separate. For example we have family court which is the lowest court and it deals with small cases like trespassing on property and disputes for child custody. the higher up court deals with theft and sometimes violent crimes. finally the supreme court deals with issues regarding things that could be harmful or things that couldn't be settled in the lower courts and were appealed

So that one branch doesn't have to much power. It's called checks and balances.