White's threatened bishop has five options, but any retreat along the f1-a6 diagonal it travelled on move 3 constitutes a loss of tempo, while Bxc6 gives a bad Exchange Variation where the lack of a fixed centre makes Black's bishop pair a fearsome force. So White is more or less forced to transpose into the Open Variation with Ba4.

This is a good example of a move-order trick. Fans of the Open Variation can avoid a whole world of nonsense (Exchange Variation, Worrall Attack, 5. d3) by playing 3...Nf6 and transposing later.

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Wikipedia has related information at Ruy Lopez