In terms of unique features there’s a few places where it shines.

Glance: You know how you open a link in the background to not lose the tab you’re currently on? Well in Zen you just click it with Alt+click and it pops open in an overlay which you can easily click away when you’re done with it (or open it to a ‘full’ tab if you need it)

Split view: exactly what it sounds like, have multiple tabs open side-by-side or above each other in the same window

Side tabs: exactly what sideberry does except the browser is fully designed around it with features like workspaces (with per workspace themes), essential tabs (are shown on top in each workspace) and pinned tabs (per workspace) all just being great.