Adding the marked stone to form the enclosure was the second pattern intensively used by Kobayashi Koichi, in the years when he was on top of the world of go in Japan. (See Kobayashi fuseki for the first.) This may appear to be a straightforward but unremarkable good play. There is a little more to it. This as a 1-3-5 strategy for Black has also been adopted as a signature fuseki by O Rissei.
In older games Black typically enclosed the corner with a high play (marked with the square).
Supposing Black got the chance to play a fourth time on this side, the circle-marked stone would be ideal: the relationships both with the 4-4 stone to the left and with the square-marked stone are beyond criticism.
With Kobayashi's choice, the square-marked stone doesn't assume a further developing play. Black may get a chance to play a later big point in the area of the marked points. With a style like Kobayashi's, this makes sense: the emphasis is on pace round the board, not large-scale play. Black is set up on this side even if White plays here first. On the other hand none of the later plays Black has here are quite ideally placed.