This is rare in pro games: the two marked stones seem to have too weak a hold on the corner territory.
This is a game Kato Masao-Sakata Eio (B) 1990-01-29. After was played the middle game started with . By this play has become natural as a checking extension.
In the game Black used the gap at a and exploited the weaknesses in the corner indirectly, to strengthen the group on the outside, rather than invade.
The requested page 464464 enclosure 3-3 invasion awaits examples.