Hey, I'm one of the cofounders of ZenHub - we're another popular integration on the marketplace and also sell our product separately through our own website.
From a company/integration on the marketplace, the marketplace has been great in terms of building awareness and exposure. Can confidently say that we've been able to reach new users and customers that we wouldn't have otherwise had if we weren't on the marketplace.
From the perspective of a GitHub user/team, lots of teams prefer to consolidate everything together on a single invoice. If you're using GitHub for software development, you're almost certainly going to need a CI/CD tool and PM tool, so why not bring everything together on a single bill?