I save mine for my Guild's quilt show, a prize is awared to the person who guesses how many empty thread spools are in a huge jar. This can be very challenging as the size of the spools varies from tiny to huge. Each year the gal in charge of this contest hunts for the largest clear jar or container she can find.
Stashbusters has a challenge to record the number of yards of thread we use up this year. So this year I'm doing double duty with my empty spools. I'll probably forget about doing this sometime mid year.
I also remember on school teacher asking for them on the quitling message boards. So they do have a purpose in their after life. May in Jersey