Why can't new input values be added to existing (used) element definitions?

Summary:

Once an element has element entries or has been processed in a payroll calculation, you cannot add new input values. My question is - why?

Content (required):

There was a change in 22B that allowed input values to be added to elements if that element, a) does not have any element entries, b) has not been used in a payroll calculation. Prior to that you couldn't add input values if any payroll process had been run after the creation date at all.

I can see many questions asking if you can add input values, but I can't see anywhere an explanation of why this restriction exists.