You are right that the clicker is basically a highly effective marker of the behaviour you want, but when you see it in action, you'll realise how powerful it is. For one, the sound of the click is much more attention grabbing for Henry than anything I can do with my voice. It means a precise thing and always sounds the same (like a whistle). Second, once you get the timing, marking the EXACT moment the dog does what you want is incredibly reinforcing.
With the heel work, you can click the instant she is exactly where you want her, and, if you chose, intermittently when she stays there. I haven't trained heel work, but I've used the clicker to really sharpen behaviours we already have. Henry had a 'lacksidasical' sit - he'd do it when he wanted, in his own sweet time. A few sessions with the clicker reinforced that I wanted an immediate sit - at first I clicked and treated every reasonably prompt sit. Then I only clicked and rewarded quick ones. And then only the instant ones. It's a great way to fade out behaviours that are only approximations of what you want but the dog has decided are good enough!