Bulky yarns have their advantages, and disadvantages. They knit up quick, but bulky yarn can be fiddly to work with. My Modern Garden Cardigan was knit in Debbie Bliss Como, a super-bulky wool/cashmere blend with a scant 46 yards per 50g. This is thick, lovely stuff. So thick, I had trouble fitting it through darning needles... or the holes on my buttons.
There are a number of solutions to this dilemma. If this yarn were made up of multiple plies, I could separate one of those plies to use as thread - but this is a singles yarn. I could find some thread in a similar shade to sew on my buttons. Or, I could find a different yarn in a matching (or contrasting!) shade. In fact, using a different color to sew on buttons can add an interesting pop to a project. But, that wasn't what I was after in this case, and my spindle was just nearby...
I decided to try and spin myself some coordinating thread for the cardigan. It would be perfectly color-matched, and wash and wear just the same as the rest of the sweater would. In fact, it would match the original yarn perfectly in every way - because that's what it's made of!
This doesn't work for just any yarn, of course - it must be a singles (one-ply) yarn. Also, the yarn I made isn't the most robust - like the yarn I spun from, this thread is soft and lofty and pulls apart with a strong enough tug. But, with enough passes through the holes of the buttons, it secures the buttons onto my cardigan well enough and it matches perfectly. Problem solved!