The principal benefit of using a build server is achieving Continuous Integration (CI). Each time a change in the code base is detected, a build that tests the quality of the newly submitted code is started.
Since there might be many developers working on the code base, each with slightly different versions, it's important to see whether all the different changes work together properly. This is called integration testing. If integration tests are too far apart, there is a growing risk of the different code branches diverging too much, and merging is no longer easy. The result is often referred to as merge hell. It's no longer clear how a developer's local changes should be merged to the master branch, because of divergence ...
