I've been wondering if this wouldn't be good for something like MarkDown editors that show the output as you type side-by-side, because you would be able to limit compilation to the block being edited (in this case, to maintain document structure the blocks would have to be be defined by headers).

Although I also would expect that dedicated tools do this already for efficiency purposes.

The reason I'm thinking about this is because I once saw a product in development at an e-learning company where they showed me a new product where (among other things) they wanted the students to write their stuff in MarkDown, in a naive "re-render the whole thing on every keypress" way. This... did not really scale at all.