24 fps is the generic movie frame rate.
25 fps is needed in some cases, for example, when you convert the video and it needs to be compliant with the 50 Hz television system, such as PAL.
15 fps can be useful in low-light conditions, as it allows longer exposure times for frames. 15 fps is also used when you select the Low light scene in the Scenes shooting mode.
You can change the frame rate in the Creative shooting mode settings.