So that they can broadcast the correct signal for your country out of their UHF aerial socket of course!
Except that no action camera has a UHF socket.
Some do have an analogue composite video output which is in either PAL or NTCS format and you need to select the format that matches your receiving equipment.
Most people use the setting to select between 25/50/100 frames per second and 30/60/120 frames per second but this really isn't anything to do with PAL and NTSC, PAL is normally 50fps and NTSC normally 60fps but some PAL systems used 60fps. Also not all cameras change the frame rate when you change the PAL/NTSC setting, the Git2 doesn't, for that camera the setting only affects the analogue composite video output - normally used for broadcasting FPV when flying model aircraft/quadcopters.