You might not think of it but todays most used mobile positioning solution, GPS, does not work indoor. But how come you still get a position when you launch Google Maps on your iPhone when you are at home? iPhone and Android phones are using mobile cell towers and wifi networks to position them selves when out of GPS coverage.
To get a great user experience and a positioning solution you can rely on - both indoor and outdoor, you need to complement your GPS. For certain usercases where battery life and bill of material costs are more important than outdoor accuracy, you can even skip the GPS and just rely on cell-id and wifi positioning.
You don't want to wait. And you definitely don't want to spend time waiting for some GPS satellites to come close enough (or to get synchronized with your phone). You just want to launch that great locationbased application and perform your tasks. Find your way, take a picture, make a Facebook check-in or similar. The GPS chip in your phone needs a significant time to calculate where in the world it is at the current moment.
Mobile positioning via cell towers and wifi is instant. The positioning information is either locally cached in your device or it will be quickly retrieved from a webservice. When you are indoor and in dense urban areas, the GPS in your phone provides none or quite bad accuracy. But by using wifi positioning in these areas you will be placed right on the spot - and instantly.