I normally add tzatziki sauce when I eat roast left-overs. I cut the roast in slices (without warming it), and I add some tzatziki sauce.
Are there different uses for tzatziki sauce? (I am sure there are.)
You can use it on its own as a dip with pita bread, use it as a spread on sandwiches, or as an accompaniment to fish and meat as you're already doing.
It's great added to flatbread wraps made with Greek or Turkish style kebabs.
I don't know about the hardcore traditional rules, but for me, I can use it on absolutely everything, and when I am in Greece I do: