An app is a PHP class that returns a tree of widgets. No frontend build, no JavaScript to write, no templates to wire up.
A real window manager
Drag, resize, snap tiling, maximise, focus and stacking, all in the browser. Your window layout persists per user and comes back on reload.
State that just persists
A public typed property on your app class is saved per user and per window automatically. Close the tab, come back tomorrow, carry on where you left off.
Ship apps as Composer packages
A third party app is an ordinary package. The host installs it with composer require and it appears in the launcher, custom widgets and all.