We estimate that by 2025, Signal will require approximately $50 million dollars a year to operate — and this is very lean compared to other popular messaging apps that don’t respect your privacy.
When you send a message, the Signal service temporarily queues that message for delivery. As soon as your message is delivered, that small bundle of encrypted data (i.e. your message) can be dropped from the queue. The storage of end-to-end encrypted files is temporary too, and any undelivered end-to-end encrypted data is automatically purged after a period of inactivity.
It also shows different service costs and why they are like that. It shows the care Signal has to privacy in its different services.