Understood. But good ideas (and the fact that you understand something) can get lost in bad, unedited writing. And in many cases, as I think I stated, it’s not clear that essay questions actually get at understanding. Rather, at least sometimes, the test memory. Can you regurgitate the facts related to X?
As for compare and contrast, yes, it can test understanding, but again it can also just test memory. But regardless of which it tests, better writing (through time to think, and editing) gives the student a better chance of showing what they know and understand.