This is a common way to develop from a 4-4 point. It is not a final enclosure of the corner, since the 3-3 invasion is still possible. The enclosure (or extension) prepares for or is played after an extension around a. Usually the upper right contains (white) influence or some solid position, so that this short, low extension from the corner, is called for.
If the left is also played out, which means the area around a contains some stones, then Black can fully enclose the corner efficiently with b, resulting in a 44-73-35 enclosure?