At first I didn't think I needed one - I had to take the thread off my machine and put it on the winder and then put it back on the machine. Pooh...
Then I realized I almost always sew in one of three neutral color threads so I have multiple spools of those colors so a sidewinder would be a great idea. Cool...
Then I figured it was just a whole lot easier to wind multiple bobbins before I start sewing. Sigh...
But, then again, I got my sidewinder at Wally World for $5.00.