For example, let's say I'm planning a trip. I will have tabs open for Google Flights, Airbnb, Trip Advisor, Google Maps and so on. A single window is one "project". It doesn't make sense to have those tabs open all the time -- just when I need them. Yet I have to keep them open because browsers don't have a way to treat a single window as a kind of persistent tab set. The best browsers can do right now is to bookmark multiple tabs as a folder and then later restore them. That's not too useful since there's no way to update this folder the same way.
What I'd like is to have windows be "named". I want to save my window as "Puerto Rico trip" and close it. Later I want to open it again and continue where I was. At all times my tabs and history are associated with the same "project".
I've been longing for this feature for maybe 10 years now. I started developing it as a Safari extension once, but ran into limitations in the API's support for opening windows.