An important psychology research breakthrough is that the ability for gratification delay is a very strong indicator of success in life, traduced in more wealth, a longer lifespan, better relationships and so on. I think the reasons for this being so aren't that difficult to understand, if you are able to delay gratification among other things you are able to:
work the extra mile for the greater good
be faithful to your partner
eat less or more healthy
restrain from addictions of any kind
To put in in few words: you cannot control your destiny if you aren't able to control yourself, self evident right?, well, sometimes self evident things are the hardest to see.