Graph analysis and phenomenon modeled by the curve $S(t)= 1/(1+e^{-t})$

Question

So the question is:

The graph looks like:

So where I stand with this question is that I am struggling to find something that it models. Once I find some phenomenon that it looks similar to, I should be able to explain the graph a bit better.

Anyone have any advice for where to look for some phenomenon that could be modeled by the curve?