Question about UDAs

[Cecil Ward]

When practically speaking would you use UDAs? A real-world 
use-case? I’ve seen them in use already for core language 
features instead of keywords like "pure", and I suppose this 
choice keeps the number of keywords down and the result is 
perhaps easier to extend. The motivation for these usages is 
clear but I don’t understand how I might use them in my own code. 
Ali Çehreli’s book mentions them briefly with an example but that 
doesn’t seem to qualify as a realistic use-case.