How does the F9 function work in Excel?

It recalculates the spreadsheet. This is particularly important when the spreadsheet is in Manual Calculation mode. F9 can be used for the manual updating of formulas. This is called Manual Calculation. Usually when you change a value, the entire spreadsheet updates automatically and all formulas are recalculated. You can change this to manual recalculation. Then when you change a value you can look at a formula and see it update when you press F9. It can let you see how changing a figure affects other cells. Doing that along with other debugging techniques are ways of finding and reducing errors. F9 will also update dynamic functions such as NOW or RANDBETWEEN for example. Manual Calculation does not have to be on to do that. Pressing F9 at any time causes the whole spreadsheet to recalculate.