More bp per battle if you win/dominate with fewer stars. Also Battle control, i.e your opponent plays first the 1st and 3rd round. (3rd typically decides things)
It's the difference between winning 11 points for the match vs winning 23 points for the match.
Knowing the rules helps: http://www.urban-rivals.com/game/rules.php?show=table
So long as they can win outright and/or get favorable round matchups, smaller hands and lower star cards get more points than high point hands and cards. People can and do play larger decks successfully, but anything over 28* is usually 1) very fast or 2) more than 8 cards, so the stars in an average hand end up being similar to what you'd get in "smaller" decks, but with some heavier hands and some lighter hands than an 8 card setup would offer.