Going back up to the top, Little Hark asked why some players . . . "play with a capo on the first fret all the time. Why? Does it make the action lower?"

Besides lowering the action (at least a little bit), the capo also makes playing easier by, in essence, pushing the frets closer together. All your stretches and reaches are easier and easier as you move the capo up the neck. I find that placing the capo at the second fret makes a big difference as I try to relearn some of the more difficult stuff I used to do. (Which in turn makes it easier to log the practice hours I need , if only to build calluses.) Even at the first fret, the capo makes each of those first-position chords and riffs a little less of a stretch.

You're right, if the guitar is "set up right' in the first place (i.e., with lowest possible action), the capo shouldn't make that much difference in regard to the action. If you play slide, though, you want a little more distance between strings and frets -- an instrument played exclusively or mostly with the slide may be set up almost too high to play conventionally, like a dobro or Fred McDowell's Sears Silvertone. So, maybe some of those guys are using the capo at the first fret when playing standard-tuning with little or no slide, then take the capo off to play slide (for which they would normally retune anyway, to an open-tuning that may or may not be a half-step lower than "normal.")