Why some transaction have send btc to same address?

[MaZyGer]

Hello

I wonder why some wallet systems send btc back to same address.

If you look to this transaction https://www.blocktrail.com/BTC/tx/5d13332abbdff9bbea02426ddaa82418e2071d91562ba999c2a8f102bd95f559 you see the input is the same address like the output last line.
Not every transaction have this but I tried blockIo and it has also same address as output.

This transaction has no same output address https://www.blocktrail.com/BTC/tx/91678f7a2452d5bc5f8164003509354397da92be6b438e8d3e30a473878722ce

Why that? Does it not make the miners fee higher?

EDIT: I am asking this because I want to calculate estimate transaction size + fee but I cannot if output is sometimes one more or one less

[achow101]

That output is the change output. Inputs are spent in full, so the amount that isn't in the output for the intended recipient must be sent to a different output that the sender controls. That other output is the change output. It is like paying for a 10$ item with a 20$ bill; you receive back to yourself 10$ in change.