Why do champs have ap scalings on abilities when there is no reason for it? For example: gangplank w and r, wukong w, Fiora w,pantheon w, and so on. What's the point of those abilities scaling with ap when clearly those are pure ad champs that dont benefit at all from building AP. Any explanation?
Because they are old designs. You won't find new champs with this at all.
Those scalings have to motivations:
Primary: They increase the effect Baron buff has on them.
Secondary: They allow off-builds. Both AP-Panth as well as AP-GP are a thing (although I don't think GP is still viable)
You mean like giving Neeko one of the highest attack speed scalings in the game and wondering why people are not building AP mage anymore as Riot intended?
Have no idea for Fiora, but Gangplank was a hybrid champ going into his rework (I remember an AP GP build back in the day) and they did plan to keep that aspect, just didn't play out that way I guess. As for Wukong and Panth they are VERY old champs. Back then AP was given to champs as a sort of way for them to deal something other then straight physical damage to counter armor to some degree. On top of that hybrid options (specificity 
when it had AP), so champs could actualy build a bit of AP and get use out of the magic damage.
Nowadays though they don't have as much value and just expose the age of the champ as much as pizza feet do.
