As for the why part, it's simple! Picture it... the glass breaks in your favorite picture frame but it's an odd size. You are doing a craft or woodworking project that require glass. The reasons you might need a piece of glass are quite plentiful, but never fear... you can cut glass at home!
Here's Jim to explain and show you how:
(Note: This method works for thinner glass - don't expect to be able to cut glass for a coffee table with this method)