Synchronous communication, indeed, is bad for the main communication channel. However, as a secondary communication channel, it can serve important needs.
When people are looking not to just get a single issue resolved, but to form a community, this community is often based on informal communication and emotions. Seeing the same people in the member list every day, exchanging little inside jokes, getting to know individual members of the community on a personal basis — this is something completely irrelevant for the user of the open-source product, but can be an important emotional component that motivates people to stay in the community and continue to put in effort into the project.
And, of course, a synchronous channel, where you can get more lax on moderation, is a much more suitable tool for this purpose.