Londoners like to know what's going on in the tunnels beneath their feet. The Tube is how every Londoner gets pretty much anywhere, and when bits of it are broken, it helps to know before you walk to the nearest Tube station.
The beauty of the Tube, (well, the central zones in particular) is that the lines are so intertwingled that it's often quite easy to route yourself around breakages or blockages. If the Northern Line's hosed, maybe the Piccadilly or Jubilee Lines might get you close enough to where you need to be. And if the Circle line's running slow (which is often is), you might be able to skip round the problem via one of the many other lines that bisect it.
Oddly, though, Malcolm had been hoping that it wouldn't appear in the App Store, while he sorted out some copyright issues with Transport for London, the body that manages not only London's transport network but also the data feeds about it that make apps like TubeStatus work. If we hear anything further on the status of TubeStatus, we'll let you know.