Why Should I Have a Chapter Narrated by a Character Over There?

"It's so tempting to do that at one point when i was doing the outlining for book three," she says. "But you really need to have a lot of reasons, and not just that, for switching p ov." The best fails are where it's quite clear to me that the author has thought, if i give a point of view to this person,  then we can know what is going on in this particular gon".