Signing a page in, or calling an API, usually means a secret: a password, a token, a key. The careless way to handle that is to paste it into the page or leave it sitting in your workspace, where anyone with access could read it.
Screenly handles it differently. A secret is delivered securely to the device that needs it, and used there at the moment it is needed. Code running on the screen can log in with it, but the secret itself never appears in the workspace for someone to find.
So you can do the powerful thing, sign a screen into a private dashboard or drive an app from a real API key, without the risky thing, leaving credentials lying around. The capability and the safety come together.