I've been thinking about this, and please correct me if I'm wrong, but I think it's a good choice for certain classes of closed source apps. For example, I'm building a video game, and I'm going to distribute it through Steam. The Linux version is going to be an appimage to guarantee maximum compatibility. Flatpak and Snap make no sense in this context. Auto updates are handled through Steam, as is integration with your system like creating icons and what not.

Also, from the research I have done (which I'll admit is inadequate), the process of building an appimage seemed more straight forward than either flatpak or snap.