* It gives you a convenient place to stash extra config options in your project. For example, instead of having to type (and remember!) -tag pkg_oUnit, it's just in the Makefile. You can't forget something that's been saved in a text file!

* It gives you a single point of entry for "other" tools which may be added later -- documentation generation, packaging, installing as a library or Debian package. I personally like all of that recorded in one file so I have to search around less.

* As an expository project, it's helpful to show off clear examples of ocamlbuild itself -- the syntax is different enough that someone just starting out in the language may not be able to grok the documentation enough to just get something running.