Patch is the complementary program to diff. Patch "takes a patch file containing a difference listing produced by diff and applies those differences to one or more original files, producing patched versions." from (http://www.gnu.org/software/patch/).
Patch is primarily used for making packages and programs natively on the slug. If you're not doing that, you probably don't need it.
M4, make, and patch should be made a part of the same package at some point. Diffutils should be thrown in there as well.