As far as understand, every time you made a transaction, (sending out), if you are just sending part of the bitcoin holdings, the excess Bitcoin will go to another address created by the wallet.
It is called changed address that you can use it or not use it. It depends on you but by default, wallet will allow you to use change addresses.
Change address is for internal movement of your bitcoins in a same wallet from one address of yours to another address of yours.
Coin analogy has better explanation for you.