Sometimes you might be in a hurry, and don't have time to adjust the diskbrake calipers correctly. Or just you can't adjust them, no matter what you do. If all you need to do is distance the rotor a tad from the left caliper, you can achieve this QUICKLY with an ad-hoc spacer you put between the left axle end and the left dropout - and this spacer you can make from a slightly thicker aluminm foil, like the top cover of butter or margarine boxes or such. It's actually a surprisingly durable spacer, it will survive easily 20-30 wheel extraction-reinsertion cycles.