Freeing up space · Temporary files and Storage Studio's own footprint
Why are temporary files explained but not offered for clearing?
Short answer: Clearing keeps a recoverable copy on your fast drive. Without the drive, that copy would sit on the same full disk and free nothing, so the rows are explained instead. Plug the drive in to clear them.
Because Storage Studio keeps a recoverable copy of everything it clears, and that copy lives on your fast drive. With the drive unplugged, the copy would have to stay on the same full disk, so clearing wouldn't actually free anything — and saying otherwise would be untrue.
Plug the drive in and those options appear. A folder can also be left out because it failed one of the safety checks — for example, an app still has it open — and the row says so where that's worth explaining. See how a temp folder is judged safe.