Since you were released from the hospital right away, you were sent home with that piece of helpful equipment. After surgery your lungs are at risk for developing problems, and breathing into your little Voldine 5000 will help you avoid this. Your surgery can cause you to favor your ribs, etc, and keep you from taking full breaths. Plus your activity is reduced.
So blow away, and keep those lungs working. Once you're up and around and getting more real exercise it won't be necessary.
It's really a pretty typical piece of hospital recovery equipment.
Sounds like you're doing well, and that you have the right attitude. Allowing your recovery to take the lead will make the process much easier. Great job!