In this case, "rehearsing" doesn't just mean playing the same song over and over again. Traditionally, that is what it means, but when you're putting on a spectacular production with a band like GN'R's status, there's more to a show than just knowing the songs you play. There's pyro cues, breaks, solos, interludes, re-arragning songs to freshen them up and so on and so on. I've never played on the same scale as GN'R, but I've been in bands long enough to know this.
that is a great observation. kudos!