I suspect the motivation behind this was to change things up and keep them fresh, but it didn't always work. Speaking of "Garden Party" and "Gettysburg," there were quite a few outings that took the characters away from the office this year, including an entire arc set in Tallahassee, Florida (more on that storyline later). Even when Andy did try to flex his managerial muscles, as in "Garden Party" and "Gettysburg," the storylines were too absurd to accept as legitimate character development, yet not funny enough to make it worth pushing the reality of the show to an ever more extreme degree. The choice wasn't out of left field, and it had some potential to bring a new flavor to the show – the insecure manager who is in over his head and knows it, rather than the incompetent manager who desperately wants to be one of the guys – but instead of properly cultivating Andy as a manager, they decided to have him share the spotlight with James Spader's Robert California, who wore out his welcome early on (as I feared he might). Season 7 ended without revealing the next branch manager of Dunder Mifflin Scranton, but the question was answered soon enough in the season premiere, "The List," with Andy sitting in the corner office. Instead of a heading in a single, coherent direction in Season 8, the stories and the characters were all over the map. Looking back at my review of The Office's Season 7, I concluded with the hope that the writing staff would plan the next year out ahead of time because having a destination in mind (in last year's case, sending off Michael Scott) seemed to have done the show some good.
Note: Full spoilers for The Office: Season 8 follow.