With our two workhorse CMS-extending plugins installed, we spend a bit of time setting them up.
"Events" on Jeff's site is a perfect example. An "Event" should have a title and description, so those default fields are fine, but it also has a bunch of other data that make way more sense as structured fields than as just text data dumped into the description field. If we apply new custom fields like "Start date" and "End date" - that a huge win-win. It's easier for us to manipulate and query on as designers and developers, and it's easier for Jeff to input (datepicker!).