tderz: if + , black has the tempo in a 3 vs. 3 liberties fight. White would wall off again and had only lost very many engame points.
Actually black could exchange a-b just before , but it might affect the aji around b-c-d.
tderz: Because at would be caught in a ladder by or , or are the local alternatives.
The 3 black stones look very funny and useless next to the white wall.