Similar to the Clay brush, but it uses a square tip shape instead of a round one.
Just like the Clay brush, it's useful for building and removing volumes and shapes like real clay, because it flattens details as you add/subtract from the surfaces.
Clay Strips is very commonly used for aggressive building of volumes and deliberate control over shapes on the surface. This brush alone can be used for a fast rough pass over the entire sculpt, with additional smoothing or polishing often required afterwards. This brush can be very versatile with varying stroke directions, repeated strokes and pen pressure to achieve various results.
If used together with Dyntopo it's easy to continuously build shapes, even in a single stroke.