when u receive 10 coins, and send 5 of em to someone else (another node),
you will also send 5 coins to yourself, without even noticing.

your node splits up the 10coin-transaction into 2, 5coin-transaction to recipient + 5coin-transaction to yourself.
you not only can, the node has to send coins to yourself.

another reason is,
someone else might have access to the address your sending coins to.
imagine services like MyBitcoin, or the markets, they all need to store your funds somewhere and can send balances from one account to another instantly (although transactions still need to get confirmed), without leaving the node.