Incremental builds. It's a pain in the ass to write this in a good, generic way yourself. If your build tools don't already understand it, then make (and similar tools) makes for a nice addition versus just a script that invokes everything every time.

EDIT: Oh, and parallel execution, but smart parallel execution. Independent tasks can be allowed to run simultaneously. Very useful when you have a lot of IO bound tasks. Like in compilation, or if you set it up to retrieve or transmit data over a network.

It's not too hard to do that in your custom script, but more care is required because until you custom script reaches make level internal complexity you will have to manually track dependencies or make sub-optimal assumptions.