Anyone who has ever played a competitive varsity sport knows that a starting spot, 'ego thing' as it may be is a pretty powerful thing.
Even if they don't play the majority of the minutes, it sets a strong tone. Tells you who the leaders are, how the pecking order sorts out, tells you how the coach expects you to play.
And after saying that, I'm thinking that starting Avery Bradley over Ray Allen might have played a way bigger role than I thought it did.