The - exchange removes a liberty of the marked group, which can come in crucial if Black descends to a. Moreover it reduces the aji of the standard placement at , which comes to life when there is a white stone at b.
Instead of strengthening his own pincered stones with a or b, Black plays from his strong stones to strengthen White's weaker stones. This is large scale ja choong soo.
And he persists, giving useless extra strength to his already strong corner, White becomes powerful at the top.