You are right that developers could build on their own machines.
But these are some of the things our build server buys us, and we're hardly sophisticated build makers:
- Version control issues (some have been mentioned in earlier responses)
- Efficiency. Devs don't have to stop to make builds locally. They can kick it off on the server and get on to the next task. If builds are large, then that is even more time the dev's machine is not occupied. For those doing continuous integration and automated testing, even better.
- Centralization. Our build machine has scripts that make the build, distribute it to UAT environments, and even to production staging. Keeping them in one place reduces the hassle of keeping them in sync.
- Security. We don't do much special here, but I'm sure a sysadmin can make it such that production migration tools can only be accessed on a build server by certain authorized entities.
