Hey, here the final goal is to retrieve a value from a Curve. For this example, lets say the Montage have a PlayRate of 2.
So, for 1sec elapsed in your game, 2sec is elapsed in montage (because it is playing 2 time faster)
So your curve is readen 2x faster, so to retrieve the last frame value, you have to read the value which is 2 frame before in your Curve.
That’s why Delta Time (which is the time elapsed from the last frame) is multiplied with MontagePlayRate. Hope this helps 