NIP-55 is a way for native apps to talk to a local signer app for login and all signing requests. It's very fast since all of the event signing is happening locally, rather than having to communicate with a remote signing bunker,like NIP-46. 

 Native apps on your phone? 

 Yes. For instance, #Amethyst is a native app, so is #Damus on iOS. You aren't accessing them through a web browser like #noStrudel or satellite.earth.

Some clients have both a web version and a native app, such as #Primal and #Coracle. 

 Thanks 👍