And why communications need to be "unified" rather than having (e.g.) separate phones, E-mail clients, and IM clients.
I think "UC" refers to customer facing more than the provision of back office technologies (although what the vendors are selling for back office sound very similar to what they tout for front office).
The problem with customer facing comms is that (increasingly) they expect you to have proper multi-channel management, such that a contact that starts as email in response to (eg) a print copy bill can be continued via voice, chat and social media applications, including access to outbound and inbound hard copy communications.
So that's a fully integrated back office technology stack, staff available to cover all of those channels, each member of staff able to see the previous contact and billing history, summaries of previous interactions, any outbound marketing (email, digitised print, possibly even untargeted media), any inbound letters or forms, and history of any self-service interaction. And all tied together with the CRM database and associated applications.
To say that is complex is an understatement, but that's what UC means to me. For large customer facing businesses, this (risky, complex, expensive, resource hungry) development is becoming an expectation - the best customer service providers can do this, and if (eg) you have a bank who can and do this, you start asking why your gas, electricity, water, TV, phone companies can't do the same. You can of course stick your head in the sand and say "not in my company", but then you need a very clearly differentiated proposition that probably offers lower cost in return for having to deal with the company on its own terms. And being cheapest in your industry is rarely a sustainable business strategy.