because if cheap tubes are used, they will not have an electronic contact with the lens, and so when the photo is actually taken, the lens never receives the command to stop down..... and the depth of field of a f/1.8 lens at that range is unusably small.
cheap tubes are £10-15, but ones with contacts are around £90 so this extra fiddling has probably saved the OP a decent whack.
P.s, great shot, focus is in just the right place and it doesn't look funny like a lot of close up coins do (in terms of annoying scratches)