Why Would You Root Stock Android?

Google's flagship phone, the Nexus 4, has already been rooted according to a post over at XDA Developers. But we're genuinely curious: why would anybody choose to root a phone that comes loaded with stock Android?

What are the amazingly useful features you can only access by rooting a Nexus 4? And are they worth the effort, when you already have a great, clean install of Android on your handset in the first place? Tell us.