I could see the use for these being a company which needs more processing power (one MSDN blog post from two years ago talked about anywhere from 400 to 2600 servers in a standard shipping container) but doesn't have the space in their existing server room.
Also handy for temporary "pop-up" sites, maybe your building burned to the ground and you didn't have a backup site, so you're renting space and put one of these in.
Back during the Christmas shopping season, my city sponsored a "pop-up" retail center in some vacant space in what used to be a vibrant retail and entertainment district. It was basically a bazaar of temporary establishments of popular retail stores from other areas of town ranging in size from kiosks to several hundred square feet of floor space. The city provided power and telephone/internet connectivity to the participating merchants. I think the whole thing was paid for by a combination of grants and in-kind donations, so the merchants only had to foot the costs of setting up, stocking, and staffing their booths. Reportedly, it was a big success among merchants and shoppers alike.
I can think of several scenarios where a "pop-up" data center could be useful, especially for businesses that for various reasons can't rely on cloud-based hosting services - a business needs to expand more quickly than it can build out the data center capacity, or a business wants to renovate its existing data center facilities but doesn't want the hassle of finding and fitting out a temporary data center somewhere else, or a business lands a new customer that will generate a lot of volume but hasn't committed to a long-term contract that would support a permanent expansion of the data center, for some examples.