Well ... I guess it depends a lot on your project, of course. If it's just your hobby project, with no releases, no dependencies, and noone but you submitting code, it might be overkill.
If, on the other hand, there's a team of developers all submitting code, automatic nightly builds will help you ensure the quality of the code in the repository. If someone does something that "breaks the build" for all others, it will quickly be noticed. It is possible to break the build without noticing, for instance by forgetting to add a new file to the repository, and nightly builds in a centralized location will detect these quite quickly.
There are of course other possible benefits, I'm sure others will supply them. :)
