With White's star point stone in the top left a white play at a would force Black into a low position across the right and build some influence facing the star point. Similarly, Black playing b, for example, would allow White to force around a. So instead Black plays a to move up to the fourth line and try to combat White's development.
Here Black allows White to play here. White is developing nicely.
If Black plays the kosumi, a simple elegant move, White's position is not so good ( at b may be better?). Black can complete the shape with a move around a.