Noob Question: I see some players in Dark Age garrisoning their villagers when herding, and then ungarrison for all the villagers to just walk back to resume herding. Why?

Title has it all. I see players do this and I don't know if there's any rhyme or reason for this.

Followup question #1: Is there an ideal spot for the sheep to be herded, or is anywhere under the TC good enough?
Followup question #2: Is there a guide/FAQ for this kind of basic stuff (like where to place your Dark Age buildings)?