We know this is good to have, but I find myself justifying it to my employer. Please pitch in on why a development team needs a build server.
It's a continuous quality test dashboard; it shows you statistics about how the quality of your software is doing, and it shows them to you now. (JUnit, Cobertura)
It makes sure developers aren't hamstrung by other developers breaking the build, and encourages developers to write better code. (FindBugs, PMD)
It saves you time and money throughout the year by getting better code from developers the first time - less money on testing and retesting - and by getting more code from the same developers, because they're less likely to trip each other up.