120 is the least common multiple of 24 and 30 (NTSC rounded to nearest integer). Perhaps this is a method to capture enough source material to generate all popular framerates.
Things get sped up, slowed down or played back with jitter. It's not great. Older devices (DVD/BD players) used to change framerate automatically. Newer devices (streaming) seem to default to 60hz with an obscure Settings menu to manually change it. Nearly everyone ends up watching the content in the wrong frame rate :-(