It's also a real app[1], independent from the phone's: after the initial key exchange, you can send/receive messages even when your phone is off
[1] Compare with the Whatsapp webapp, which solves/sidesteps the E2E encryption among multiple devices conundrum by simply routing everything through the phone. The Signal app is also written with web technologies, so it might not be palatable for everyone, but it's a good compromise imho
If you open them in a browser when you're offline, you won't be able to load/read your messages (yes, I know about HTML5 manifests for offline data... but that's a mess), but with Signal you can.
Moreover, being able to vet/verify the updates (which you can apparently even block altogether) before running the app is of paramount importance for a secure app like Signal.
With a run-of-the-mill webapp that's also impossible.
Again: tradeoffs. I'd prefer if Signal desktop was built on something different, but I still happily use it as is everyday.
https://whispersystems.org/blog/signal-desktop/
It's also a real app[1], independent from the phone's: after the initial key exchange, you can send/receive messages even when your phone is off
[1] Compare with the Whatsapp webapp, which solves/sidesteps the E2E encryption among multiple devices conundrum by simply routing everything through the phone. The Signal app is also written with web technologies, so it might not be palatable for everyone, but it's a good compromise imho