It's used to enter the formula as an array formula. Array formulas are used when you have a function that normally applies to only one cell (in this case, Isnumber) that you want to apply to a range of cells. It forces the formula to take each case individually according to the functions and convert them to TRUE/FALSE or values as appropriate.

I'm not really sure why Microsoft deemed it necessary to add this step, but oh well...That's the way it is.