The frets are fanned to allow for a longer scale on the bass side and a shorter scale on the treble side of the instrument. This solves a few problems:
1) No longer will the B-string sound darker and less distinct compared to the other strings.
2) The G-string retains warmth and normal tension compared to extending the scale of all strings.
3) You will no longer have to adjust your attack depending on which string is being played due to the more even tension from string to string.
4) You will no longer find the tone adjustments for one string interfering with another due to the more even tone across the strings.
5) You will no longer be unheard and ignored by your audience due to the combination of the longer scale low strings and the even tonality across all the strings.