It is a horizontal rail that allows you to keep balance during your home training and can be easily moved around the house. If you’ve ever used a chair in order not to tumble down during your exercises, you know what I mean.
The barre is also more advantageous because not only does it look better than a chair, it is usually designed in a way that neither you nor the barre could tilt during the workout, unlike the chair that can easily slip from under your hand.