Hi,
you may create timer job when you dont need any user interface:
1. Send Report or Data from sharepoint site to user at specific time or day such as Daily / weekly / Monthly / Yearly
2. If you have to do some operation at every minute or at some specific frequency
3. If you have any schedule task or activity
Here is some good example to create timer job:
http://dotnetfinder.wordpress.com/2010/07/24/creatingcustomsharepointtimerjob2010/
http://code.msdn.microsoft.com/sharepoint/SharePoint-2010-Custom-416cd3a1
Hope it could help
Cheers, Hemendra-MCTS "Yesterday is just a memory,Tomorrow we may never see"