As Eugene (and Gertrude) note, varying your right hand position along the length of the string can produce different colors, appropriate to specific effects you are trying to achieve in the music you are playing. You might, for instance, play a repeated phrase first with the plectrum over the scratch-plate, then repeat it near the top of the sound hole for a different color in addition to a different dynamic.
Another thing to note is that as your left hand moves up the fretboard into higher positions, the string is made functionally shorter (this is especially notable on the mandolin). It often makes sense to move the right hand as well to keep the impact of the pick at the same relative location on the string. In other words, in a higher position I might play with the plectrum well over the sound-hole... not to achieve a sul tasto sound... but to keep the sound color consistent with that of lower positions.
All of this points to having a right-hand technique that does not "plant" the arm in an imovable position on either the bridge, or the body of the instrument.
"The effect is pretty at first... It is disquieting to find that there are nineteen people in England who can play the mandolin; and I sincerely hope the number may not increase."
- George Bernard Shaw, Times of London, December 12, 1893