Marginal bets started when first online casino was opened - they are invented not to give money, but to get money from referral system - if you remember pop adds(they are still here) - "Try my new method and become rich in ten days, just sign in and money will come"
Deal was that you sign in, deposit some minimum of 20-50$, take 20-50$ bonus etc and you try marginal with casino - as the owner of pop-add is rich you would become too
. After playing lots of red-black, you realize that after only 10 bad strikes you are betting 500$ to win just 1$. After losing you realize that marginal don't work in long run if you play, but you also find out that there is way to earn with marginal in long run - with referring. So, you find people who will sign in with your ref link, you take 10% of their deposit, they lose, you tell them something went wrong, try again you will win, they deposit and bet again, you take again 10%, they lose - you are actually telling people to lose their money so you can earn some.
So, don't fall on marginal tricks, who ever think marginal is way to earn, please, think twice! Because it's not!
Cheers!