I mean that (some of) the older engines play weak but still coherent moves, with a more natural (dare I say human-like?) flow to the game, making them easier to learn from. If you “cripple” a strong AI to bring it down to 10 kyu level, some of its moves will be just random nonsense.
Having said that, I’ve not yet tried the latest KataGo upgrade: maybe that does something better on a low rank setting?