Many prompts START as formulas, but will be overwritten with either modified formula's or hard coded values as you adjust the product to what you need in a particular instance.  But its not uncommon to realize you changed something that you should not have changed.  And the only way to get the value back would be to place a new instance of the product and pull the formula from that.  It would be nice if when a product is saved to the library, that its initial prompts are saved somewhere...like over in those columns IW+.  Which would give you the ability to revert changes to any prompt in a product.  Would also allow some safety checks similar to the checks done when you save a subassembly.  "This value WAS a formula when the product was brought in, but its a hard coded value now" or the formula has been changed.