Why do you need to find an oxidation number?
Well, because they enable us to balance complex electron transfer reactions, redox reactions, quickly and straightforwardly.
I grant that the use of electrons as actual reactants in a redox reaction is a bit abstract. But their use, to balance presumed electron transfer, allows us to balance redox reactions in a systematic way. The difference between oxidation numbers is the number of electrons conceived to have transferred between oxidant and reductant. Think of the assigned oxidation number, and the electrons transferred as convenient little fictions. The fiction does conform to experiment; and if it didn't it would not be used.
See this old answer.