What's the reason behind manaless champions?

First, I'm not trying to say they are OP or "noob" champions or anything like that I'm just curious as to why some champions don't use mana. I thought maybe because they need to spam their abilities constantly but let's be honest Cassiopeia exists and uses mana. So I'll just ask here if some of you guys know or can link me a page where riot already explained it(if they did)
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