Some functions, like LEFT() are both Excel functions and VBA functions.
Others, like Excel's SUM(), have no VBA equivalent, but can be called from VBA using WorksheetFunction.Sum or Application.Sum
If you look at the Members of the WorksheetFunction Object, you'll see that many, but not all, Excel functions can be called from VBA.
For example, you can't call Excel's MAX or MIN functions.
Also, be careful, there are differences in the way some functions behave between Excel and VBA. Even though VBA has a Round function, I will often use Application.Round so that my results round the same way that Excel does, rather than using VBA's "Banker's rounding".