Each time I put a bootable drive in the usb slot, it asks me if I want to install it. Can I use it to reinstall my OS without formatting my harddrive?
Please correct me if I'm wrong - I haven't used Ubuntu in a while since I moved to Arch Linux.
I believe Ubiquity (the installer used in the Ubuntu LiveUSB) has the ability to reinstall Ubuntu over a previous install of Ubuntu while retaining all personal documents, pictures, music, etc. - in a nutshell, your home folder.
On the "Installation Type" screen during the install process from an Ubuntu LiveUSB, it should show an option along the lines of
Replace Ubuntu <version> with Ubuntu <version>. These can be the same versions, and should preserve the contents of your home folder.
Note: This will not save your installed programs. If you're looking to do that, check out this question.