If we are being honest, we would admit that admirals are cool and all but there is nothing interesting about them. The yonko are the leading pirates after the One Piece (which is what the story is about), knows a little (or a lot) about the history, have weird bodies (BB, Kaido and Big Mom).
We will learn nothing about the One Piece world with Akainu backstory or Kizaru's one.
Only interesting Marine is Garp because he is the father of the revolutionary leader and one the most famous and infamous pirate. How did it happen, who was Luffy's grandmother and mother. What happened between Garp and his son?