Callisto runs in your browser and as a desktop app. The browser gives you the assistant; the app gives you the assistant plus your whole computer.
Free to download, works with the same account, and everything you make syncs back to your browser.
Why so much is app-only: a web page can't take a keyboard shortcut while you're in another program, watch your screen, read files off your disk, or reach devices on your Wi-Fi. Those aren't gaps waiting to be closed — they're the reason the desktop app exists.