Apps · The Apps window and launcher
Why do offloaded apps still appear in the All Apps grid?
Short answer: So everything you own is in one place. Offloaded apps can't be opened from the grid; right-click and choose Restore to This Mac to bring one back.
So you can see everything you own in one place and bring an app back without hunting for it. An offloaded app is archived on your fast drive, so it can't be opened from the grid — clicking it only selects it.
Right-click it and choose Restore to This Mac to bring it and its data back, or Remove from Fast Drive… to delete the archive to the Trash.