Tiny example, you get a mail from a co-worker, in Outlook you can see if he's at his desk, and start chatting or initiate a call (voip OR PBX phone) with him to ask about an unclear part using the integration with OCS.
Then you can schedule a meeting, still in the same application, and make sure everyone you invite is free at the time - and also book your favorite conference room four with that awesome projector and not the dinky one that can't handle greens in conference room three.
Half of the people you invited are on the road with Iphones, Androids and Winphones and got the meeting request in real-time. One of them is new and looks you up in the company directory directly from the same phone app to figure out what department you're from. Realizing you're his new boss he rejects the invitation with an additional comment about your scary hair-do.
You quickly flesh out the vague agenda in the invitation to a fully-fledged project site on your internal Sharepoint and send out an update for people to review this before the meeting. While editing the agenda, another person see that you're doing that right now and are probably not interrupted by a question about it and initiates a quick text chat to ask about bullet number four which she thought was a bit insane, and you answer on the spot, proving your insanity.
In the meantime, a system administrator is moving your mailbox storage away from the old servers without you really noticing, using a simple Powershell script based on today's coffee which is vanilla flavoured.
Getting closer to the meeting you quickly check the statistics out in Outlook, these days there would be too much noise having personal messages about attendance so most invited people just accepted or rejected the request. This time, no-one did the rude "suggest another time" button and with two exceptions everyone seems delighted to attend your awesome meeting - so you email the coffee machine to prepare your regular triple espresso while walking across the office.
As someone said, you can call your mailbox as well... never really got used to that myself though in this smartphone world.
The presences works fine in the web mail interface as well these days, which is rather cool.