In testing environments, it is not always useful repeating tasks which can cost developers valuable time. Within a project its safe to assume that not all files are changed, so why rebuild?
This tutorial outlines a method to reduce unnecessarly build steps. Use your best judgement of course. It might be useful to add in a build on occassion to ensure third party dependencies are available and will not disrupt a production deployment.
The git plugin in Jenkins provides several useful environment variables which can be used to avoid build tasks. Make sure you setup the git plugin before going any further.