Well, if you are thinking of modern display devices, they will usually up-scale the frame rate to their native display rate. Good display devices will have a native frame rate of 240 hz. This allows natural multiplication of both 24fps and 30/60fps signals. Some other devices have 120hz framerate, which is not the best for movies. Older CRT projectors could do a native framerate of 48Hz, but nothing really less.
The 24fps would be to keep the natural frame rate of the movie. Otherwise, you’re looking at up-converting it to 30 or 60 fps, which can cause motion defects (judders, etc.).
