There are a couple unused megabytes after partition has been wiped I was trying to erase my partition and reinstall OS X but it says that 108.35 out of 108.50 gigabytes are being used, I'm just curious on what they are used for and why is there space still being used even if the disk has been wiped.
Every drive is different when it comes to the amount of space that it takes for it's own use.

For me, I have a 128GB SSD in my MacBook Air, but if I look up the amount of space available to me it usually shows that I have 120-121GB of actual usable space.