What's the point of this? Matrix, Signal, and Telegram all have open source clients, so you could make a client that used the native protocols and not route you

What's the point of this? Matrix, Signal, and Telegram all have open source clients, so you could make a client that used the native protocols and not route your messages through this third party's servers. Like Pidgin, Miranda IM, Trillian, and others. But I guess a monthly fee and cloud services are the more modern approach.