Similarly I find it very useful for if I've written a tool script and really don't want to write the command line interface for it.

"Here's a well-documented function - write an argparser for it"

...then I fix its rubbish assumptions and mistakes. It's probably not drastically quicker but it doesn't require as much effort from me, meaning I can go harder on the actual function (rather than keeping some effort in reserve to get over the final hump).