What's the benefit of delaying a tenure-track position for doing a postdoc (when you already have a tenure-track offer)? I've seen candidates for tenure-track positions in computer science that managed to negotiate a starting date that is a year later than originally planned. During that additional year, they all seem to be doing a PostDoc in various places. 

I'm wondering what the benefit of this is. Many departments provide you with a lighter teaching load during your first year, so it seems to me that the overhead with moving to a new place (for just a year), working on a postdoc salary, and then, before the 12 months are up, moving again, doesn't strike me as an efficient way to develop one's career. 

Is there some big advantage of doing such a postdoc rather than immediately starting your tenure-track position that I'm missing?
Once you have a t-t position, you're in a race against time to get as many publications as you can before your tenure clock runs out.

A postdoc effectively gives you one more year of focused attention to research and publications before you have to start teaching. 

Note that teaching can be highly disruptive to research and publications, especially in your first year.