One reason for retuning in old time fiddle tunes is that
they use a fair amout of cross tunings (i.e. "open tunings)
(AEAE, AEAC#, ADAE, ADAD, I've even heard D'DAF#). This isn't
just for laziness in fingering; not only are drop-notes
and drones possible without having octopus fingers and
perfect pitch, but even if the string is not bowed, you
get harmonic resonance in these strings which changes
tonal quality. This really adds to the "sound" of the
old-time fiddle, where the tunes are more centered on
the key and not given to the flights of fancy that are
more common common in the bluegrass realm.

In other cultures, open tunings are more common as well.
Norwegian hardingfeles have not only open tuning, but
additional sympathetic strings for even more resonance.
But there they do take to "flights of fancy" at times,
modulating the key (and the rythm; they came make a
3/4 on paper into something approaching a 5/4 time) and
even getting into semitones and such. Perhaps the drones
and sympathetic strings serve to remind them how to get
back to where they came from. . . .   ;-)

I suspect a lot of the cross tuning tradition in old-time
fiddling comes from its closer relationship with its
prior roots in Celtic music than is true of bluegrass.

Cheers,

                            -- Arne Langsetmo