They were creators whose entire business models were based on this. I gather there were also a third party integrations, companies that actually developed against the Patreon API so that their own services could be integrated with creators' campaigns on Patreon – that is to say there were companies that made products that they sold to creators, that relied on the API.
Well Patreon decided that they're not supporting the API anymore. Apparently, from the screaming on the (now long defunct) creator forums, for a while there it looked like Patreon was going to turn it off. The Patreon walked that back and said that they wouldn't turn the API off, but they wouldn't be supporting it anymore, and they wouldn't be doing any further development on it.