What is the purpose of a dedicated "Build Server"?

These machines are used for several reasons, all trying to help you provide a superior product.

One use is to simulate a typical end user configuration. The product might work on your computer, with all your development tools and libraries set up, but the end user most likely won't have the same configuration as you. For that matter, other developers won't have the exact same setup as you either. If you have a hardcoded path somewhere in your code, it will probably work on your machine, but when Dev El O'per tries to build the same code, it won't work.

Also they can be used to monitor who broke the product last, with what update, and where the product regressed at. Whenever new code is checked in, the build server builds it, and if it fails, its clear that something is wrong and the user who committed last is at fault.

For consistent quality and to get the build 'off your machine' to spot environment errors and so that any files you forget to check in to source control also show up as build errors.

I also use it to create installers as these take a lot of time to do on the desktop with code signing etc.