Supremely handy for wheel building. Enables you to shorten any spoke and re-thread the end. Clamp it in your vice, clamp a spoke, turn the handle and hey presto a newly theaded spoke.
It is easier to use if when shortening spokes you leave some thread, make the existing thread longer and then cut the spoke to the final length. This works well if you're shortening spokes up to 8mm.
I got my Hozan Spoke Threading Machine off Amazon for around $150.