ELI5: Why do nuclear power plants have massive circular towers?

I'm not knowledgeable on these things but I believe what you're referring to are the cooling towers, no?

They're there to help maintain the temperature of the reaction in the core. If you don't keep the temperature down, it can start to runaway, which is what leads to a meltdown.

But as I say, that's mostly guesswork. I don't really know what I'm talking about.