It would be really useful to be able to calculate values from user inputs using basic equations. For example, to total up the numbers a user has entered in several fields to calculate a total price. The result could be a visible text field or inserted into a hidden field.
My use case is that I'd like to calculate where the user's answer is of a numeric field, between the minimum and maximum allowable value. E.g. answering 5 on a 0 to 10 numeric input would give 50%.