What's the point of leveling masteries?

I'm a bit new to the game and I've just looted my second mastery 6 token for Warwick. It seems that I can use them to unlock said mastery rank, by using both shards and a chunk of Blue Essence. But I can't see why I should do it? What is justifying me spending half the price of a champion? Does it give skins? More loot when playing the champion? What exactly does unlocking masteries 6 and 7 for Warwick allow me to do?
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