When we place a wager,we are not trying to pick winners.We are saying to another party that we are more right than you.
If I thought that Kingston Town rated as an Even money chance but bookies were offerring only $1.60,then I should make an effort to lay the horse,even though I thought it will win.
To make money you don't need to pick winners,just be more right than the opposition.
If I had a credit card I would be straight off to one of those Betfair things.Not for the seemingly fairer odds,but the opportunity to lay a horse.