I'm a fairly novice spinner and therefore have not plied a lot of yarn. For those of you who don't spin, the first stage of spinning fiber produces what's called a single. Twisting two singles together, or plying, makes 2-ply yarn, three singles makes 3-ply, etc. Making 3-ply requires 4 bobbins: 3 for the singles and one for the take-up. I have exactly 4 bobbins for my wheel but one is usually full of practice spinning or another project. Bobbins for my wheel cost $30 each so it doesn't make economic sense to have a large supply of bobbins around. And besides, it'd be too tempting to abandon a spinning project to start another one if I had a lot of extra bobbins. I'd never thought about winding the spun yarn onto something else until I came across this blog post:
Reduce, Reuse, Recycle!