I think mostly people who pray before betting or gambling a certain game is like believing some higher being above us all listens and hopefully side with them when they talk to Him. It keeps the peoples faith too. And there is nothing wrong with praying even if you are betting or gambling, in my opinion it all boils down to your prayer if it is heartily being delivered or addressed to God. But you know, to each his own. I know a few people praying and making the sign of the cross before they bet on something like cockfighting or even betting a boxers game, for example a Manny Pacquiao’s fight.