In 13: 14-17 the Torah discusses the asic form of tzara'as and says, "He will come to the Kohen...and show the Kohen..if it changes to white he will be pure..the Kohen will declare him impure..etc"
This is a very simple form of tzara'as. Some forms are complicated;different colors, hairs, on clothes, on the walls of the house, etc.. but the form here is very simple and the splotch on the skin is either white or fleshy. What is the big deal that the Kohen is needed for this?
A person has to defer to authority. it could not have been too difficult to recognize this splotch and identify it as either pure or impure - most people probably could have done this one on their own.
But no. ובא אל הכהן- he comes to the Kohen. He has to respect the authority of the Kohen and defer to him and allow the Kohen to do his job.