The biggest reason to join a MCN would be if Adsense isn't yet offered in their country or if they need to be paid via Paypal. Outside of that, the vast majority of what is currently offered from a network could be found for free or at a minimal cost.
You're correct, they don't promote channels. They do, however, often offer various tools or a new YouTube dashboard that shows more information and gives recommendations on tags or descriptions. Instead of joining a network, download the free (or inexpensive depending on the options needed) TubeBuddy extension.