You’ve got a 20-yard pitch to a tight pin over a little bunker. You reach for your lob wedge, give it a good swing, and advance the ball 10 yards into the bunker.
Why?
Because you were pitching from an upslope, which added loft to your 60° club, launching the ball nearly straight up in the air. From an upslope, a PW, 9I, or even an 8I might have the same effective loft – 60° or so – as a lob wedge.
The opposite is true on downlopes, of course: your pitching wedge may have the loft of a 7I. Take a loftier club to counter the negative effect of the slope.