I have voted for this feature as I believe it’s a great addition.
When the PoP functionality was first introduced we raised the need for %age changes, and I believe this is the natural next step.
As @mabr notes, absolute changes are sometimes not useful at all. Is a change in 5 ok? What about 10? It doesn’t have the same business context. However, as @mabr notes, a change in 50% which depending on the filtering etc. can be 5 or 1000 can be significant for a business.
Say for instance you want to measure the change in sales WoW. Sales can be seasonal so a drop/increase in 100 sales might be significant in Winter when your sales are lower but in summer this is a small change. You wouldn’t want the alert to fire on the absolute change, instead a 10% change would be a much better indicator and (from my experience) what businesses would be focussing on. Rarely do businesses look at the absolute due to external factors and seasonality.