Why are there brushes on the sides of escalators?

Unfortunately, they're not there to clean your shoes - they're there to keep your feet away from where the step meets the side of the escalator. That's where many of the worst escalator injuries occur, so it's best you forego your customary between-floor shoe cleaning (I write to myself, begrudgingly).

Schindler, an international escalator manufacturer, explains their trusty elevator skirt brush here: A continuous length of bristles projects out from the skirt, gently deflecting and guiding riders away from the step-to-skirt gap. The brushes are made with durable monofilaments, and provide a soft surface to warn passengers and encourage them to keep away from the extreme sides of the step. So...can you keep brushing your shoes with them, even a little? No, stop that.