This toggles the visibility of the inspectors.

If you've moved the inspectors around and you want them back in their factory-set locations, this is the menu item for you.

Okay, this takes some explaining, but once you have used it for a couple of minutes it makes perfect sense. Inspectors that are currently visible and expanded appear with check marks next to them in the menu. Selecting an inspector from the menu does something based on the current state of the inspector: a hidden inspector appears (along with any other inspectors grouped with it); a collapsed inspector expands; an expanded inspector collapses.

There is one trick specific to these menu commands and their associated keyboard shortcuts: for your convenience, if you collapse the only expanded inspector in a group, the whole group gets hidden.