As GrumpyGramma stated, I too have used it mostly for shoulder joins.
But just recently, I learned to use it for joining large sections of knitted work...like joining a two or three sections of a scarf...and joining long strips of blanket squares.
I picked up stitches along each long edge to be joined...and then worked the 3-needle bind-off to join the sections together. I really like it. It seems to be stronger than weaving or crocheting, and it's less bulky than the mattress seam.
All of these mitered squares were joined using the 3-needle bind-off:
And just last night, I used the 3-needle bind-off to seam three panels to one another.
(The item is an Art Deco Wrap, and only Right end and Middle Panel in the photo.)
The 3-needle bind-off is quite useful!