Pouring over the release notes regarding jQuery 1.4, I came acrosss $.noop() which is:
Description: An empty function. (added in 1.4)
The only logical reason is if you're calling a function that does something AND calls another function, and you want the higher-level function to do its thing without calling a parameter function.
Most of the jQuery functions optionally take a parameter function, so you don't have to pass one in. Maybe there's one or two where that's not the case -- or maybe it's to assist developers with their custom code that behaves like this.