In 2.79 and earlier, we had a panel inside the sidebar called 'Item'. In here you could rename objects inside the viewport:
Riggers liked to have a fast way to rename things directly in the viewport.
We removed it because it was quite arbitrary and out of place. But when we did that, we thought to add a popup operator to replace it, like so:
To make it fast, the text would ideally be selected when calling this popup, so that users could simply make it appear, then immediately start typing and press Return.
This is similar to the M-key popup for Collections.
We just never got around to adding this. Therefore, this task is here so we don't forget.