Functional groups in organic chemistry are tied to the oxidation state of the key C in that family of compounds. Recognizing the functional groups and the oxidations states at C helps one to remember the chemistry linking one functional group to another. For example, oxidation of primary alcohols produces aldehydes. Reduction of the aldehyde takes one back to the alcohol family. Reduction is just the reverse of these.
Nomenclature pays special attention to oxidation state.
If there are more than one oxidation state of C represented in a molecule, generally the one in the higher oxidation state takes precedence for naming purposes. Of course, complex molecules may have two or more C's belonging to different families.