Insomnia is useful if you are running a logging application that you would want to keep working while your phone is locked and the display is off.
If you are using an IM or IRC client that you wish to stay connected to receive messages while the screen is off.
If you are the creator of a logging or IM application hopefully you can use the Insomnia source to build the ability to stay on into your application, without harmfully deleting power config files. e.g. ApolloIM and MobileChat. However if you would like your application to do something periodically a much better approach would be to schedule a wake up, and allow it to sleep in between times, to conserve battery power. Email me and I can show you how to do that."