People have chosen to keep many mutations and defects in breeds for some odd reason, and keep breeding for them - like extra toes, misshapen feathers, holes in the skull, etc. all because they think it "looks cool", without a thought for what that means for the bird itself and its quality of life. It's the same for other animals as well - cats and dogs with crooked short legs or flat faces that make it hard to breathe. They get the mutations because they tend to be horribly inbred, and then people choose to keep breeding for that mutation intentionally, and make it part of the breed description. People are cruel and selfish beings.
Your chicken has extra toes either because it was inbred, too, or because it has some extra-toed breed in its lineage, like silkie. Sometimes the extra toes pose problems, sometimes not. Chickens weren't meant to have that many toes. I am ethically against propagating defects in animals, so I would not own any breeds (of any animal) where that line has been crossed, just as a matter of principle. But that's just my decision.