- Be able to revert to a previous version of an application, either because my current working copy is corrupted or full of unanticipated bugs.
- Sharing the project with other developers for collaboration or contribution.
- Maintaining multiple branches or tagging code at specific milestones.
On why you have no excuse for not using source versioning:
http://betterexplained.com/articles/a-visual-guide-to-version-control/
http://www.relisoft.com/co_op/whyvcs.html
http://www.mactech.com/articles/mactech/Vol.14/14.06/VersionControlAndTheDeveloper/index.html
http://jamesmckay.net/2009/06/why-would-anyone-not-use-source-control/
http://www.soundsoftware.ac.uk/why-version-control