Privacy matters. To have privacy on the internet, we usually rely on encryption. Signal is one of the best tools out there for end-to-end encrypted communication. Yet encryption is not always enough. We also need to take good care of metadata.
What is metadata?
Metadata is the context of your message. From whom to whom, when, with what device, which app, etc.
Metadata is very important. Some even said that "if you have enough metadata you don't really need content". Indeed, knowing who knows who is already valuable information, and can be enough to infer a lot of information, including political views or other sensitive information.
Weak attackers
Someone sitting in the same coffeeshop, sharing the same wifi hotspot and looking at network traffic (he is a tech enthusiast) wouldn't be able to know more than "This person is using Signal (actively or not)". Which you can argue is already sensitive information, it already tells something about Alice. For that, you can use a VPN (but they're not silver bullets either).
If you're communicating with someone also sharing the same wifi hotspot, he would also be able to know "This person is probably communicating with that person".
Strong attackers
But we also need to consider stronger attackers, including Signal themselves, the people hosting them (which would be Amazon in the case of Signal), people owning part of the internet infrastructure, or even a collusion of such actors. We consider such powerful attackers in provable security.
In the case of Signal, message content's privacy is proven in such a strong model, but for metadata, we don't really know.
Such strong attackers can gain even more information through passive observation (I see that when Alice is sending a message, Bob is receiving one). They can also actively interact to gain some information (If I block Bob, Alice is not receiving a read receipt).
Conclusion
Yet, Signal is still an essential tool that we should use as much as possible, but please remember that it's not a silver bullet. Such metadata gathering could even transform Signal into the very mass surveillance machinery it tries to circumvent (the same argument can (loosely) be made for other encrypted messaging apps like Matrix or (even more strongly) for Telegram, WhatsApp…).
What we really need is metadata-free communications.