solder will crack under stress - such as when the wires are moved as
they are pushed back into a box. solder will also weaken and detach
under thermal loads - such as if the wires are undersized and get warm.
the wirenuts also provide physical protection to the connection and
the sharp points/edges (wire ends).

note also that many modern wirenuts are designed to be used on wires
that are not pretwisted - many will say this right on the package.

Rusht Limpalless wrote:
I was always taught the 'right' way to use wire nuts was to pretwist or
'pigtail' the bare wires first, apply the wirenut in correct size and then
use vynyl tape over all as insurance.
Why not just solder?? Never given a straight answer by anyone except that
its not neccessary or allowed. I see they also have wirenuts that
incorperate both twisting and crimping after application, anyone had any
experiences with these?
Sorry for complicating a simple matter, but i like doing right and knowing
why its right, not just blindly adhering.