This article talks about how Foursquare leased more office space than they need in Manhattan so they could have the option to expand if needed later one. In the interim they have subleased space to a number of startups.
This is a great example of a startup community that helps each other out. Typically when you sign a lease you are required to pay a security deposit equal to 2-3 months rent. Well, in New York it's common to have to pay up to 12 months of rent for a security deposit. Most startups can't afford that. Subleases are important for the survival of the technology communities as it allows them to sign short term leases and pay less up front for security deposit.
This is just one way that some companies are able to give back to the startup community. We need to do more of this in Austin, Tx!