It literally is the most secure way to communicate out there, period.

It’s the most secure reasonably popular and user friendly way to communicate. By having unique identifiers and a central server a determined state actor could still figure out who is chatting with whom, and when (but the content of the messages is secure)

There are other protocols out there, like SimpleX or Briar, but they operate on a threat profile way beyond most people need, and they are less intuitive than Signal