Yes, but if I want to make larger modifications than would make sense for the core project, I'd like to have the ability to self host my modified version (and ideally have a support contract as well, if they're into that).

So you asked what the point of doing this is for them, from a business perspective. I think the point is marketing / smoothing the sales process. I feel much better about using SaaS products that I know I can self host if necessary, even if I'm unlikely to actually do so.

Frankly, it's just the same reason I prefer any of my tools to be open source. I don't like using proprietary programming languages or frameworks, because I can't fix things that are broken even if I want to. This remains true even though I can count the number of times I've actually done this on one hand.