Bookies want to make money, not break even :grin:
No matter who wins. (the book is said to be "round")
Ideally, they would like to make the same money no matter which player wins, ('square")
Which is why the odds change, to attract bets on some players, and discourage bets on others.
The thing about betting with a book is that you get the odds at the time you placed the bet - you know what your payoff will be.
With pari mutual betting, you get the odds when betting closes, so you never know what the payoff will be when you place a bet.