Why is Gangplank's E so overloaded?

I am curious why GP's Powder Kegs have so many things attached to them: 1) Procs Sheen and can crit 2) Splash damage 3) Ignores 40% armor 4) Slows enemies 5) Self speed buff I could get over the fact it does a lot of damage, since it's a freckin explosion and it's what makes Gankplank a teamfight monster. But why both a slow and a self speed buff? Remove one of the 2 components. He can either comit to killing you or run away, depending on the situation. It's Jinx passive and W combined on a few seconds CD. Shift more power into his ULT by giving it an AD scaling, but nerf that barrels ability please.
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