Good point. There is more room for schmancy fancy stuff if you're preparing a performance. I like fancy schmancy.
...unless you are the one trying to come up with those stuffs ;P. Then it starts to become a headache.
I think my creativity is strongest at "modifying" things, at least in dancing. I am really bad at coming up with something completely new, yet working really well with the music.
Feedback from judges is always good. So that is a plus.
I also know this from a personal experience, and from people that I watched, preparing a showcase helps you improving on the said dance. For instance, at one point of time, my friendly rivalry and I (and our partner, of course) went to a comp. Me and my partner only won the chacha. And during that semester, they were working on a chacha showcase, so at the next competition, they won the chacha...but we got the rest