The object ID in PLOGI is used for what?

Hello, I'm back for some little questions.

We know that every employee has a personnel number. And in table PLOGI there'e a kind of records whose obj type is P(Person). I'm wondering what are the obj IDs of these records used for? Cauze we already have PERNR to stand for a person.

This question also lies in Company Code(Obj Typ - IC), Personal Subarea(Obj Typ - I1) , etc.

Thanks.