(Please don't take offense if that's something you've done! I'm sure there are reasons that make sense to the people involved - I just can't grasp the idea of doing it myself - doing a second ceremony wouldn't hold any meaning to me.)
Abe suggested that it's about renewing something that fades over time. For example, you might get a tattoo re-inked because it has faded over the years. Or recommit to your faith. I countered that yes, you may be making an official step towards renewing something that was intended to be permanent, but it's not done with the same fanfare as the original, nor does it take as much work or commitment to renew as the original did.
I assume re-inking a tattoo takes much less time, and you don't have to plan out the design and do an outline and fill it in - you just re-do the fill color. And although you may recommit your faith, you aren't re-baptized, or publicly reaffirm it in the church (as far as I'm aware in any Christian variations), it's just a quiet, personal confirmation of your previous decision. Similarly, even though, of course, your commitment to the marriage may "fade", in the sense that the strength of the relationship will ebb and flow over the years, why should recommitting to it take the same form as the original commitment? It's simply not the same thing.
Granted, I also don't really get the appeal of ceremonies in general. I guess it's that for me it has to hold a lot of significant meaning to be worthwhile, and I often fail to see what makes a particular ceremony worthwhile. For example, I didn't participate in my undergrad graduation ceremony, because it didn't seem like something worth celebrating, as an accomplishment. Sure, there were some aspects of the journey that were difficult, but growing up it was drilled in me that I would go to college, there was never ever any doubt in my mind that I would graduate. Why spend the time celebrating and ceremonizing something that was just a given?
I also am not planning on attending a graduation ceremony for my graduate degree - I finished the degree last summer, but the school only holds a commencement ceremony one a year after the spring term. I do see this as more of a ceremony-worth accomplishment, but the timing is off, and becomes unconnected with the event. I had the option of attending the ceremony last spring, prior to when I had completed my final class - but that seemed pointless, there isn't anything to celebrate yet! What if things go wrong and I don't finish? Or I could participate in the ceremony this upcoming spring. But then it's practically a year later. I'd rather celebrate in my own way now, than participate in the prescribed ritual when I've moved past it.
What do you think about ceremonies and rituals? Can they hold meaning for you even if repeated?