Well, it does.
Option A : A large, well-known, brand name agency with a long term track record of success.
Option B : A small, relatively unknown firm with a short track record.
Let’s say both performs well. How do you make a choice?
Option A if :

You need ice-breakers at those corporate cocktails
You like your account managers to change every season
You like to brag on how much you spend on your PR people – somehow “I only spent a 120 big ones on my PR is not as glamorous as saying I spent …