I use them instead of virtual desktops - each with a specific hotkey, and some with customized pinned apps.
I have …
General: Email, shopping, etc.
Gaming
Media
Two Work activities - a primary, and a secondary for when I need to compartmentalize different ongoing tasks
Other - for anything transitory that doesn’t fit in the others.
I realize this could largely be done with virtual desktops, though I don’t think you can have a different pinned app loadout for each?
The downside to setting things up this way is when I restart my computer, it seems to randomly decide which browser windows go in each activity. Also, with apps that I use across them (like Notion), I have to go hunting for which activity it opened in. To get around the issue of splitting Firefox across different profiles, I just use two browsers. Firefox for work, and Firedragon for personal stuff. They share the same external password manager, so it’s pretty seamless.
I mentioned this in my own top-level comment, but I just use different browsers for work and personal. Firefox for work, and my distro’s fork for personal. That keeps those nicely separate.