Hello!
The purpose of the 3 branches is to dedicate certain responsibilities to the specific branch. for example, the legislative branch makes laws. the other 2 do not
the purpose of checks and balances is to allow each branch to prevent the other 2 branches from being too powerful. for example, the president can "veto" - or stop - a new law that congress has made, before it is put into effect
I hope this helps, and have a nice day!