I wrote about some of mine here:
DT’s unique file IDs have helped me in a couple of creative ways. Most recently I used it to maintain a study/flash card system as I studied for PhD comprehensive exams. I continually use files stored in DT as resources for automation, too, e.g., as templates for populating journal entries or notes or whatever.
I also keep a lot of references in DT. I find the search rewarding if you put a little bit of effort in. It takes no time to pull together a variety of items about a given topic from many folders/groups. A colleague recently asked me for some topics and I was able to push a dozen simple papers to them from years ago without much ardor.
One more for good luck: I have put together an auto tagging system with DT3’s smart rules that adds 10 Finder-compatible tags to every new file based on the DT-proprietary concordance keywords related to the file. I’m scheming to find similar uses of smart rules…