I've bought a travel bag and there is a piece that came with it that I'm having hard time figuring out what it is used for. Here is the picture:
Any idea what it's used for?
@Michael and @tohecz are close... but not quite there.
It's not actually a luggage tag... rather it is a protector for your luggage tag.
It works like this one:
However the rounded shape is intended specifically for the luggage tag that comes on some Guess luggage, such as the one shown here:
I suspect your bag came with a similarly shaped luggage tag.
The long slit allows you to reveal the luggage tag for reading, then easily slides back in place.
Why a luggage tag protector is important is beyond me.
But I believe that's what your mystery item is.
I guess that you can put a label with your personal information there, so that the finder knows who the bag belongs to in case the bag is lost.
I would agree with what Michael says: it's for your personal badge. Typical usage is that you put inside a heavy-paper card with your name and home address. Typically, you put the name there in such a way that it's visible when the paper is inside (so that you can identify your luggage), and you put your home address in such a way that it's covered (so that nobody can get it easily, but still the luggage can be shipped to you if you lose it).
However, I am yet a bit dubious because these things are usually attached to the luggage quite firmly.
I believe it is a credit card holder, wallet. The slot is there to make it easy to push the card(s) out of the wallet.
I found an example of something somewhat similar that might help confirm my guess.
The main thing that makes me unsure of my guess is the rounded shape... I would expect a credit card wallet to have square corners to hold the cards snug.