Beeper provides a free service to bridge all your chats into the same place, including traditionally secure applications like signal with end to end encryption. By using beeper, you are letting them decrypt all your signal chats and re-encrypt them on their servers. I wouldn't trust a paid service with my privacy on this level, much less a free one. An alternative model could have installed the bridging and stuff directly on your device in the app, but from a usability standpoint that becomes less convenient especially when trying to port all the chat applications to all the platforms. They are just a hosting service for open source bridges with a nice closed source client.