> As if you can't get a whole lot of information on most people with just their phone number. The number of people whose Signal ID is built off a burner phone ad no longer traceable back to them is miniscule.

Yes, but what are you going to do with this information? All you know is how long they've been a signal user and when they last connected.

You correlate that with the chat logs you've secured from a phone that's been confiscated or subpoenaed.

The metadata itself is just as valuable as the content of the messages.

If you want to prove that criminal A was in correspondence with criminal B, that's how you do it.

As per this comment, they store much more than just the last connection time[1].

[1] https://news.ycombinator.com/item?id=39445791

If you got the physical device and the data on it (unencrypted), then what do you need the server for?

You're not thinking this through. You might have someone else's device with access to their signal chats, but need to confirm the identity of someone they're talking to. You might have been able to ID a person but only have had temporary access to the message data (eg undercover agents who sneak or are granted a look at someone else's Signal messages). You might have a Signal conversation with someone you suspect of crime, and want to establish correlation with their use of signal (by most-recently-accessed timestamps) and some other activity.