I... don't get activities. They seem like a slightly enhanced workspace/desktop system, but the isolation is slightly higher and I never got the hang of it.
Think about it like having two separate user accounts on your system, but instead of having to log into each one separately and switch back and forth by going through SDDM, you can instead just tap a keyboard shortcut or however you have it set up and switch between a "work" and "personal" mode, or a "gaming" and a "normal" mode. You can have different apps pinned to your task bars, change layouts, etc.