Ruy Lopez C64 (I'm curious)
The black bishop is in a really exposed spot if it cant be traded off, which it can't after kf1. White has qa4 or in some lines qb3 coming, and black doesn't have an adequate answer.
{EDIT] While I was writing this, Quickerfinger showed that he had a quicker finger by making the same point first!
Art's explanation is good, as usual but one thing that hasn't been mentioned is that White has some threats to win the pinned Nc6 with d5. And that knight is the only thing protecting the Bb4. So one plausible sequence might go: 7.Kf1 Nf3 8.d5 a6 9.Be2. Now is Black saves his knight with 9...Ne7, then 10.Qa4+ picks up the Bb4.
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