Because if you have been using a conventional washing powder (most brands have a significant amount of fillers) OR a homemade laundry detergent with soap and insufficient vinegar or citric acid to ensure there’s no residue, you might have quite a build up that you can’t see in the hoses of your washing machine.
I have actually heard of one case where someone switched to Norwex laundry powder, and instead of doing a machine wash, she started out with a load of whites! Disaster! The higher quality laundry powder washed her machine, but she ended up with the gunk all over her whites! Oh no!
How to do a machine wash
Start by chucking an old towel, cloth nappy or similar in a very hot wash with a cup full of vinegar and a scoop of the Norwex laundry detergent (and the magnet ball if you have it, and if you are using a top loader).
The towel will pick up all the gunk pulled from the machine. Sometimes you may have to do a couple of washes (particularly with front loaders as you can’t use the magnet ball in them) but it will all be worth it because the Norwex detergent and magnet ball will keep your clothes AND you washing machine cleaner!