Many kids gave in to the temptation and ate the marshmallow and others waited for 15 minutes and earned two. The kids that participated in the experiment were monitored for the rest of their lives (and in fact are being monitored even now) and it was found that the kids that did manage to wait for 15 minutes have been consistently doing better than the ones that didn't in almost all aspects of life.
Quoting Wikipedia again:
The children who waited longer, when re-evaluated as teenagers and adults, demonstrated a striking array of advantages over their peers. As teenagers, they had higher SAT scores, social competence, self-assuredness and self-worth, and were rated by their parents as more mature, better able to cope with stress, more likely to plan ahead, and more likely to use reason. They were less likely to have conduct disorders or high levels of impulsivity, aggressiveness and hyperactivity. As adults, the high delayers were less likely to have drug problems or other addictive behaviors, get divorced, or be overweight. Each minute that a preschooler was able to delay gratification translated to a.2% reduction in Body Mass Index 30 years later.
It is amazing how one skill has been found to be associated with so many good things in life. On the other hand, it also makes sense if one thinks about it for a while. It happens way too often in life that we want to get something but we do not want to go through the procedure required for getting it. For example, people like cleanliness, but do not like cleaning; they like to be healthy but do not like exercising or controlling their diet or regularly visiting the doctor; they like to get good grades in exams but not the studying required to get them; and the list goes on. Most people end up acting upon their immediate whims instead of what's good for them in the long run. This is exactly the case of preferring a small reward now over a bigger reward in the future. People are spoiled with aggressive marketing of short-cut options like wear a belt to reduce weight in 10 minutes; learn tricks to solve mathematics questions within seconds; join some coaching for guaranteed increase in marks etc. It makes sense to do that if you live in 10,000 B.C. where any long term plan could be unexpectedly foiled by being killed by a lion. However, in today's world, where at least statistically speaking, life is much more certain than it used to be, it is clear that people who act based on long term plans will do better than those who don't. You should thus prefer bigger rewards in future over small ones now, because most likely, very soon you will be in the future facing the consequences of the decisions you took in the past.