TLDR: Adds keybindings for inspecting your target and mouserover target, which are missing from the vanilla client.
If you want to check someones gear, with just pressing one key... well, you're out of luck.
The "vanilla" client does not provide the keybindings for inspecting a player.
You have to do it "old school" by clicking on the player and then right click and select "inspect" or you install this little addon which adds the necessary keybindings to the game.
Just open up the keybindings and click on "InspectKeybinding".