The open G fingering with the middle finger on fret 3, Low E string, comes out of the primary scale fingering pattern for scales starting on the Low E string.

Obviously, in most circumstances, it is the preferred position, or at least the starting place. The Guitar allows(maybe requires is a better word) you to play the same notes in a variety of fingerings and positions.The same is true of chord fingerings. If you are playing in the key of C, the fingering that uses the ring finger on the low G can be very useful. However, if you are playing "Carter Style" in the key of G, it will be very awkward.

There are three to five basic fingering positions for playing scales (two are considered to be variations by a lot of players) and you have to pick which works best for what you're trying to do--same goes with the chords.