While the phone app allows to lock the application with a passcode this is currently not possible on the desktop.
From time to time, I give friends the permission to use my PC, e.g., for managing a music playlist or for quickly looking up something on the internet.
Right now, everyone with access to my computer can open Chrome and look at all my synchronised conversations in Signal. I would feel more comfortable if I could, just as on the phone, set a passcode to unlock Signal on the desktop.

(I don't know anything about how Chrome apps store data but maybe this could as well be used to encrypt the database itself.)