I suppose techniques like vibrato (which you can't do on an open string) and harmonics (which you can normally only do on an open string) make a clear distincti

I suppose techniques like vibrato (which you can't do on an open string) and harmonics (which you can normally only do on an open string) make a clear distinction and give a composer an incentive to use a particular key (or give a player an incentive to use a non-standard tuning).