I've only redone a handful of wheels,
but i've noticed when using new spokes,
that the thread revolutions are usually even and equal.
It really makes cinching it up easier since you can count tightening revolutions and depend on them to be truer. (moreso anyway)

The few i've done with used spokes have enough stretched or twisted that it adds alot of unnecessary time making it true.
I guess its good practice though!

Also i've never re-used stainless steel spokes which probably don't stick like the old galvanized type when removed,
you know there's always one.

TP