When Black plays 3...a6 to set the Morphy Defense in motion, you, as White, face your first major crossroads in the Ruy Lopez. Do you preserve your proud light square Bishop by retreating to a4?
Or do you give up the Bishop pair in order to rupture Black's pawn structure? The second option comes by way of 4.Bxc6. This line is known as the Exchange Variation. Black can recapture with either pawn though 4...dxc6 is more common.
There are pros and cons. The good stuff is that it is a less risky line and you will be harder to beat. Also there is much less theory to sift through and learn.
The not so good stuff? The loss of the Bishop limits you somewhat as an attacking force. It's a little harder to drum up a potent attack.