Using the capo to emulate hi-boy string/tuning for Wild Horses. I did this to compliment a video by privettricker, who does great Stones lessons. They were discussing the highboy tuning and whether or not is was done, and by whom, on the record. He demonstrates the G tuning that Keith does.
I used to do a solo act, and it helps a lot to get a fuller sound and complimentary voicing when using the capo.