A .bin file has many uses. It is used by many virus scanners and anti-spyware software as an update file for example.
One of the main uses of .bin files right now is the creating and burning of CD-Images. When the CD is completely ripped to a .bin file, a .cue file is also created that contains information on the .bin file for burning or mounting software. A .cue file is also created when a .bin file is actually made from files on a HDD and not from a CD.
If you were to look through your hard drive, you would probably find a lot of files with the .bin extension, but don't be fooled, these are not all CD Images and they are most likely very important to your operating system or software that you have installed.