The reason chords can be voiced (fingered in different ways) in so many ways is basically because there are some many ways to play a G maj chord. Why so many and why be concerned? Because in "voice leading", each note in a chord must progress melodically to the next note in the next chord.

I'll try to explain. In every chord there are four voices (regardless of doubling). These voices are called bass, tenor, alto, and soprano. So each note in a G major chord will take a voice. Since there are only three notes in a G major chord, one will be doubled. If G is in the bass it will be doubled in another voice. So if you assign a note to each voice you have this:

G major chord
soprano - B
alto - G
tenor - D
bass - G

tabbed for guitar:
e---7------
b---8------
g---7-------
d---5-------
a-----------
e-----------

Now say you wanted to go to a D major chord next. Voice leading requires that each voice line (except the bass) move the smallest interval to a note of the next chord.

Take the D major chord (D F# A)

To move the smallest interval possible, here's how the chord would be voiced.

D major chord
soprano - A
alto - F#
tenor - D
bass - D

SO going from G major to D major, the correct voice leading would be.

e---7---5---
b---8---7---
g---7---7---
d---5---0---
a-----------
e-----------
G D

In voice leading, each line in the chord has it's own melodic progression. Making a great sounding harmonic progression. If you go by every possible progression of chords, you will find hundreds of different voicings. That's why there are so many.