Explain WETH to me?

So, often when buying tokens on Uniswap or other DEX, you first have to swap ETH for WETH. I thought maybe this was because the token in question wasn't an ERC-20 token ie not run on Ethereum but now I'm not sure. If a token wasn't on Ethereum then why wouldn't you just buy it with whichever token was relevant, like BTC or SOL etc? Also I'm key sure I've had to do that with tokens that are erc-20, but I could be wrong about that part.

Ok, that's pretty much my question, but now I'll have to pad it out a bit more... So is it something to do with the way DEX's work, maybe? Like, can they only work with ETH so if there's a token that's not on ETH you have to use WETH? I'm assuming WETH is wrapped ETH so it can run on other chains but maybe I'm wrong about that too. Anyway, I know you can swap for other tokens on dexes so I guess it can't just be ETH. Explain me!