A person uses nPr when the order matters, and they use nCr when order does not matter or for combinations. Someone would use these functions when performing probability math problems.
Probability math looks at the likelihood of something happening. Not all events have a predictable outcome, so someone uses probability to essentially predict the outcome. For example, if someone flips a coin, when the coin lands it will either be on the tails side or the heads side. Since there are only two sides of a coin, there is a 50 percent probability that the coin lands on heads, or a 50 percent probability that it lands on tails.