I mean, for a start the messages are stored locally, it doesn't protect the messages server-side, because they are never stored on the server. All the pin does

I mean, for a start the messages are stored locally, it doesn't protect the messages server-side, because they are never stored on the server. All the pin does is keep the server-side data, i.e. your address book and conversation information, safe. Secondly, outside of encryption, it should not be Signal's job to enforce whether a logged-in user of a phone can see that user's content. It is the operating system's job to enforce user-level security, and users of Signal are unlikely to keep their phones without a pin, fingerprint, or other level of authentication. The fact that you cannot turn off the option to lose all your messages when you lose a pin that you have been forced to set is horrific for those of us that rely on conversation history as a memory aid, and is precisely the reason why I have refused to set a pin.