Attack speed helps with clearing camps, and the defenses help sustain a little. With scaling ability power you gain more than just having flat AP.

I have been playing Diana for a couple if years and when she started to fall off after last years worlds, i have been trying to come up with another build for her, until I picked up Ekko. I noticed that a top Ekko build was made that was really off and being used in Pro play so i decided to make a Diana build somewhat similar.