Splitting Frequencies is an important element in getting a fat ominous sound. Spreading the sound in the stereo field without losing punch, adding FX chains to different bands. It's all possible with frequency splitting. Imagine using a bass line or other bass heavy sample in a project. You might wanna lose the rumble but excite the mids adding more harmonic content. Adding more harmonics is easy with distortion but you don't want the rumble to get excited too. By splitting the frequencies over 3 bands you can add any FX chain to your Low, Mid and High Bands.
In Ableton this is really easily achieved via the Audio Effects Rack. Simply click on chain By clicking on the middle (striped) icon. Now you see the panel where you can set up the different chains. When you frag and drop a Effect a new chain is being created. Once the chain is in place you can in turn drop more effect on the individual chains.
Drop the Multiband Dynamic or EQ on there. Repeat this process 2 times. This is the foundation from where you can start splitting the frequencies. By adding a EQ Eight or Multiban Dynamics to each chain you can completely control which frequencies are being passed trough. Set them in such a way that you won't create any artifacts; meaning you don't want the bands to overlap. Reader Bert commented the most accurate way to achieve this is by copying the exact frequency values for the cutoffs into each instance of the EQ's. With the Multiband Dynamics all you need to do is solo each band - Hi, Low, Mid.
It's also a good idea to add a utility plugin on the low and high channel. Setting the low band to mono and the high band on wide. Leaving the mid channel on stereo. (100% merged - 50 -50 % or 100% separated). In utility this translates to 0% on the bass chain and 200% on the high chain. This will create a bigger wider sound, without losing the punchyness in slow waves.
It's a big time saver. And, maybe even more important (and indirectly another time saving aspect) it stops your projects from getting cluttered and you'll find yourself bouncing less sessions over a longer period of time.
So go and enjoy the split loveliness.