A low cut filter makes room for your bass and kick.

Your vocals, guitars, snares, and countless other instrument tracks will introduce some unnecessary frequencies into your tracks.  Using a low cut filter on those tracks will free up room for instruments whose primary frequencies are in those areas.  Your kick, bass, or low end synths won’t have to compete for space with your violins or vocals.

Your low cut filter will clean up the bottom end of your mix.

If you’re recording a 4-track demo, these unnecessary frequencies may not really be a big deal.  However, if you are recording a 24-track song, those unnecessary frequencies will start to add up, creating a nice mess to interfere with your bass getting through.  A low cut filter on all your mid and high-frequency instruments will allow your low end the headroom to really deliver its sound.