You use the LINEST function to perform a regression analysis.
And you perform a regression analysis when you need to know, for example, how an athlete's performance is affected by age, height, and weight. You can then use the results to predict the performance of a new, untested athlete. In other words, you're estimating likelihood.
As an example, say you have sales data from January to June, and you want to predict sales for September. You'd use the LINEST function like this:
Remember to enter LINEST as an array formula (press Ctrl+Shift+Enter instead of just Enter).