In arrangement view, currently you can only split at the "insert marker". It's common to want to split during playback at the current position of the playhead. So you're listening along to your arrangement and you'd like to split it up by ear via cmd+e while it's playing. Currently not possible.
Ableton devs, if you don't want to decouple split from the insert marker, then an additional shortcut for "split at playhead" would be very useful. Splitting while playing is standard in other DAWs.