Perhaps some Agents feel selling the story to an Editor is a bit easier, when the same person who has written it, is the person who fleshes out the story with the Illustration as well. Submitting to an Editor a dummy, vs text alone. This may be even more true with books that have very few words and were the story is being mostly told through the art.
But I do think the biggest draw for the Agent is, that advance and royalty split. If they are repping an Author/Illustrator, the full advance goes to that one client and so does, eg:, 10% royalty, as opposed to an Author only getting just 5%.