The main use case would be getting into trouble with the authorities. However, there are a few sub-cases (most of which will still in part fall under the first

The main use case would be getting into trouble with the authorities. However, there are a few sub-cases (most of which will still in part fall under the first one) a) Educational - you can learn a lot about how telecom networks actually work by playing around with your own BTS. However, it would be advisable to do so in a place where your emissions don't interfere with other, legit operators - say, in a mine or something. (I am not being flippant here). b) Nefarious, non-state: it is trivial to trick any compatible cell phone into connecting to your rogue BTS instead of one belonging to the victim's carrier. This could be done transparently to the victim - ie. you forward his call data to the network at large, MitM-ing him and monitoring his conversations. c) Nefarious, state actor: Much the same purpose as b), though presumably a (legit) state actor would be able to just serve the telco with a warrant to get at the same data - the exception being, of course, if you were operating on someone else's turf - say, you are some intelligence service operating abroad, eavesdropping on another state's principal actors, for instance. d) Fun (closely related to a) above) - say, in particular if you operated in an area with no effective RF licencing regime (failed states and offshore springing to mind), you could effectively become your own cell phone carrier, for instance while hosting a conference on a vessel in international waters, allowing participants to use their handsets to get in touch with each other. This option could be utilized either with or without a gateway to the global phone network.