One of the biggest concerns in escalator design is preventing objects from becoming trapped between moving and stationary components.
Although modern escalators are built with strict safety standards, there is always a small clearance between the steps and side panels. If soft materials such as loose fabric are pressed into these spaces, they may become caught.
Engineers sometimes refer to this as an entrapment risk. The side brushes help minimize this risk by discouraging passengers from leaning against the side panels or allowing objects to rub along them.
Many transportation authorities around the world include these brushes as part of their overall safety strategy, alongside emergency stop buttons, warning signs, step markings, and routine maintenance inspections.