Clicking the item bar requires a repeater region. The built in usage is with hotkeys which you define in the WoW Keybindings menu. As Ualaa said, it can be used for any kind of item.

I've used it for quest items, potions, food, hearth stones, equipping new gear, learning new pets/mounts, opening the All Hallows handful of candy and the bags from the Headless Horseman and a bazillion other things.

Macros can be used in the item bar, however, they need to be in the same location in your WoW Macro panel. For example, I make a mount macro and call it "Mount". Because of how I've got all my other macros named, "mount" sits in slot 7 on my main, slot 3 on my healer, slot 3 on dps1, slot 4 on dps2, slot 6 on dps3. If I drag that mount macro to my Item bar, it will call whatever is in slot 6 of my macros panel for everyone, so only my main is going to mount. My other guys are doing things like dropping Tremor Totem and trying to cast CC, because that's what they have in that location. But, if I rename the macro to "A", making it the first macro slot for everyone, then they all use the proper macro. It works the same with summoning Companions.

However, you can drag a spell directly from your spellbook and the location doesn't seem to matter. For example, Judgement is in two different "slots" of my spellbook for my Ret and Prot paladins. But if I pull Judgement out of my spellbook and drop it on my bar, everyone gets the Judgement Icon in their bar and both my ret and prot cast Judgement. If I drag Arcane Torrent {the racial} out, all my Blood Elves will use it. If I pull out Autoattack, everyone will use it. It works this way with professions, as well. If I drag out Jewelcrafting, it opens the JC panel on my JCs and nothing for anyone who doesn't have Jewelcrafting.