Do junk DNA is like code styling and comments in programming.
Also disclaimer: My only knowledge of this is from Nessa Carey's The Epigenetics Revolution and some additional online reading.
Basically it's like using a "buggy filesystem" (like ext2) that only shows the first ~16000 files in a directory. So having too much junk can hide stuff, having too little of it can uncover files that evolution carefully hid, and so on.