What can I do in a browser, and what needs the desktop app?

Signing in, the store, your library, purchases, clubs and chat work in both. The difference is everything that touches your computer:

• Only in the launcher — downloading, installing, patching and launching a game; choosing where games are installed; playtime and achievements that depend on a game actually running; the built-in wallet; and, for studios, uploading build files.
• Only in a browser — using the Creta Wallet browser extension. It is a browser extension and cannot run inside the launcher, which has its own wallet instead.

So you can buy a game in a browser, but you play it from the launcher. When a browser page needs the app it shows an Open launcher button rather than failing quietly.