Instead, it means "to give you an image to rest your eyes on, so reading all of these annoying letters doesn't make them catch on fire" or something.
This is so weird.
Definition of illustrative : serving, tending, or designed to illustrate
illustrative examples
art that is illustrative of provincial life
"Hitler was a really bad man. Here is a picture of Hackernews user `kenneth` for illustrative purposes."
Something like that happened in France a few years ago. A TV channel illustrated a news segment about cybercrime with the first B-roll of computers they could put their hands on. It turned out to be shots from a segment on a not-for-profit org teaching computer skills to the poor (including former inmates). The org was pissed, the TV channel just said it was "for illustrative purposes", as if that was justification enough.