What is it?

You're on the right track with stringed instruments, but it tensions nothing.

 

Does it start--or facilitate--a string's placement on the instrument?

 

It has nothing to do directly with the strings (only tangentially with them).

 

And we have a winner.

It is literally tangential to the strings!

 

Nice work, Red! I always thought that frets were machined to different lengths, then set in place. (I learn something new every day).

 

Fret wire comes in spools or long lengths that luthiers press into neck grooves, then nibble the ends and file to the ideal height and shape:

 

Yeah, I was feeling charitable.