You deskktop app could login to Lemmy via the web app and store the login cookie as it's token for future access. This security is effectively on-par with the existing web app in terms of what happens if the machine falls into bad hands.
But the same thing via an API would be preferred.
The idea that you can login to a website and get a cookie that last 3 weeks may feel absurd, but when you think about clients keeping unencrypted passwords it sort of makes it more appealing comparitively. Especially if you can lock down the cookie to the hardware to prevent theft somehow.
..but does it taste good?
Ultimately it helps to understand the benefit of the command line: That text is a more effective mechansim for communicating that anything else. That any command you learn can easily to turned into a script an automated. That commands can be copied and pasted and shared with friends much easier than videos or images.
Look at the explanation in Windows for how to change a registry key or how to change a printer setting. It's one long guide full of screenshots thats painful to follow or understand. Where-as Linux users can easily share commands and fixes or tests over a simple irc chat, because the command line reaches the whole system.
The command line is of course a place where lots of apps can be plugged together and mixed up to achieve hundreds of goals. awk, grep, sed, wc can all be mixed together so counting users on the system. At this point in my answer I realise I am just regurgitating things written on thousands of webpages on the internet!