Stretching is often overlooked; but in barre you need to lengthen to strengthen. And this runs true for muscles in general.
Everything has it's proper place, your muscles included! To efficiently tone a muscle it can't be cramped up in the wrong position and unable to move in it's full range of motion. In the same way it's inefficient to try and find your favorite top in a messy and unorganized closet. Things need to be in order to not only move organized but look organized too.
Basically put; the foundation for long, lean and gracefully sculpted muscles is the flexibility to move each muscle from start to finish.