The program unify has the purpose of manipulating context diffs and unified context diffs. unify will accept either a regular context diff (old- or new-style) or a unified context diff as input, and generate either a unified diff or a new-style context diff as output.
Various other options allow you to echo the non-diff (comment) lines to stderr, modify the diff by removing the comment lines, and/or tweak the diff into a format that is good for releasing patches.
I think most people prefer unified context diffs in general. But some of us just have trouble reading unidiffs, unless they get very simple. Usual context diffs show how the code was before, and then, how the code is after. Some people just prefer understanding twice thoroughly, than once fuzzily. The tool is useful for those who handle a lot of diffs from various sources, and want them in a uniform format.