That is exactly what I do with the two open runs in The Chicken Palace - I close one off and sow it with meadow weeds and then open it up for them and swap over to the other one.In worst case scenarios, there can be 5 different sections. I doubt so many will ever be needed but I can imagine growing weeds in one section at a time for the chooks to graze on, and I can imagine using sections when integrating newcomers.
I have discovered they destroy a run full of weeks very quickly!