This was a recent mandolin cafe topic too. Don't know if these are the real reasons, but they're reason enough for me.

Can't say I notice them much on guitar, but on mandolin the 10th fret marker on one pair of strings is the same note as the 3rd fret marker on the next higher pitched pair of strings. This facilitates like fingerings across the strings in locations seperated by 7 frets.

The 9th fret 3rd string marker on guitar is the same pitch as the open 1st string, same goes for the 9th fret 4th string and the open 2nd string.

Are you talking about 4 or 5 string banjo?

Peace, Mooh.