Just in case you are seriously wondering why you have to connect your phone, it's because the web interface is basically a convenient interface for your phone. WhatsApp works exactly the same way. It works pretty well.
Presumably it is because synchronizing conversations across devices while using end to end encryption is really difficult. You could give up end to end encryption but I'm pretty sure you wouldn't like that either.
Why do I hate to give up end to end encryption? My end is PC, encrypt that. If I want a shared message history, it's understandable that I have to perform some kind of synchronization between PC and phone to, but it should be optional feature. I don't even want it, personally.
If you get to use it just your personal computer (any modern OS) without a smartphone, than you would also get to choose your own identifier. For Facebook (WhatsApp) this is a non-starter, because your phone number is a way better identifier to link your profiles and sell your eyeballs to advertisers, so it will stay a requirement.
On a PC you could even have multiple accounts on the same device, which is not good value for advertisers either (one device per person makes it easier to correlate various data sources and track you).
No idea why Signal doesn't enable us to use their tool without a smartphone. It makes it look a bit dodgy from a software freedom perspective.
Presumably it is because synchronizing conversations across devices while using end to end encryption is really difficult. You could give up end to end encryption but I'm pretty sure you wouldn't like that either.