Bell curve or more widely known as the Gaussian distribution/function is a theory that states how a standard deviation happens in any environment and by doing so this theory proposes a two or three dimensional graph that shows a succeed line that starts off with exponential function, has a top point and then decreases in exponential fashion. This Bell curve is a very widely used graph to show different environmental, statistical and other phenomena and study their speed of change as well as where the peak value could be. If you would search around for this specific graph this you would start noticing it more and more and see that this normal distribution is a part of almost any process starting from sales and new product introductions to population growth as well as oil and other non-renewable energy source availability and peak values. As I understand this phenomenon then a bell curve can appear in any situation where there is a limited amount of something but an entity(most likely humans) are not acknowledging this and doing everything like there will be continuous growth all the time. This phenomenon can be seen in the peak-oil graphs that you can find where some statisticians and data scientists have predicted that because Oil is a non-renewable resource and we as a species are using this resource like it will never end then we will soon get to the point where demand will exceed supply and would have reached the peak of the curve. When this happens then there will be no point in trying to find new oil fields or new extraction methods because it will be more and more harder to do so.
This normal distribution curve has a formula that states how wide will the curve be and where the top point resides and then the curve is created from these values visualizing that distribution to anyone and showing more insights than a simple data-set could ever do. These bell curves are used for a wide range of applications and they show up in almost every statistical data where there is a real world experiment with data gathering. Almost always there is a standard-deviation curve that will show some underlying truths about the data and will allow the statistician to make predictions about the standard deviation and overall data sets. Quite frankly there is not one application or any reason why these bell curves appear in data but they show that underneath all datasets and all experiments are some overall truths that we can extract.