Collaboration and a whole bunch of other stuff. BTW I work @ Cisco Systems.
A third party developer has gone ahead and developed a Android client for Office Communications Server called AndrOCS. Not to sure if its compatible with Lync but its currently in beta and downloadable from here.
Together with an open source Pidgin plugin (for Linux), a third party iPhone client from Modality, a Blackberry client from RIM, and of course most famously the (now tested/validated/qualified by Microsoft) snom300 phones, this illustrates that Microsoft's speech about really being open is not just idle talk - it is fully feasible for third parties (using the protocol documentation freely available on MSDN and/or the UCMA platform) to build feature rich clients for OCS or Lync. There is just no equivalent story from the traditional industry vendors.
Please try aLync, a new IM client for Android. It can connect to Microsoft OCS/Lync, MSN, Yahoo! Messenger and Google Talk. You can do file transfer, text editing(bold, italic, copy, paste, search), create a conference and many more in a unique tabbed interface. You can make calls and send SMS messages to your buddies, simply by making gestures on your touch screen.www.alync.net