Rail-creep is caused by the rolling-wave effect from the wheels. The rail is pushed forward in the direction of travel. Except on steep gradients, where gravity causes the rail to creep downhill.
It is important to prevent rail-creep because it can cause expansion gaps at rail joints to close up. Which can cause the track to buckle in hot weather.
The link shows a photo of rail anchors in use on USA spiked flat-bottom track.
When used on bullhead track, the rail anchors bear against the side of the chairs.