To answer your original question, they don't need butter, but having butter as well really enhances the taste so is nice to have, rather than useless fat and calories.
To answer your follow up question When you all say butter, is that proper butter or I-can't-believe-it's-not-butter pretend stuff, there is a special place in hell for anyone who calls anything except actual real butter 'butter'.
It's not butter, it doesn't taste like butter, so don't fucking call it butter because I am sick of having to watch everyone who serves me with food like a hawk to make sure that they are using actual butter and not some shitey pretend stuff.
A recent example: I wanted tea and crumpets in the Asda cafe, which I'm sure everyone will agree, need actual real proper butter and not spread under any circumstances (I'll let off vegans and the dairy allergic obviously).
Cafe woman takes my crumpets out of the toaster and reaches for massive white tub. Excuse me, I politely interrupt - is that butter? Yes, she says. Me Is it actual butter? Cafe woman shows me the lid of catering pack of shitey vegetable margarine saying 'yes its spreading butter' to which I reply (still being polite but seething inside) I don't want that, don't you have any butter. Cafe woman says 'yes it is butter' to which I reply, I don't want that I want proper butter to which she replies 'sorry we have run out'.
I am now resisting the urge to shout, this is a fucking supermarket cafe, there is half the EU fucking buttter mountain not ten feet from where we are standing how the fuck can you not have any fucking butter but obviously I didn't because I am a nice person who knows that people who work in supermarket cafes have to follow ridiculous pointless petty rules and thinking for themselves or showing initiative even to make a customer happy is generally frowned upon. So I didn't have any crumpets and just had my tea and sat there thinking that if they hadn't already made my pot of tea, I could have gone into the supermarket and bought my own butter to take back into the cafe for my crumpets, but they would then have moaned about that too. Wouldn't happen in Waitrose
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Obviously very much a first world problem and yes I am very lucky if that's all I have to worry about 