HTTP itself doesn't give you any standard way to synchronise blobs of real-time data (IMs, VoIP signalling, etc) between servers, or even really between servers

HTTP itself doesn't give you any standard way to synchronise blobs of real-time data (IMs, VoIP signalling, etc) between servers, or even really between servers and clients. Sure you can write a webapp which uses some combination of long-lived GET, PUT, websockets, HTTP/2 Push etc to 'push' realtime messages around the place, but there's no standard way of doing it; each app makes up its own fragmented and proprietary set of APIs. So Matrix fixes this by defining a standard HTTP API (which can also be used with other RPC transports if you prefer).