I used to live in an earthquake-prone area, and we always kept a moderate supply of non-perishable food, water etc. Plenty of other people didn't. It's easy to say 'oh, it'll never happen', or 'if it DOES happen, we're all fucked and your tinned beans won't make a difference', but there's a lot that can happen between those extremes.
And it's the same with Brexit. If we have a difficult few days or weeks, I'd like to be self-sufficient, even temporarily, and be able to provide for myself and my dependents. It's not sustainable, but during an unstable period it prevents you being a burden on others and reduces the amount of time/effort/money you might spend chasing difficult-to-find goods.
It's worrying that people apparently believe only speciality or frivolous products will be affected if there's no deal. Then again, I can't understand how anyone who's kept even slightly informed about our politicians' self-interested, incompetent and directionless shitshow of the past few years can possibly say, with any genuine faith, that things are going to be fine.
And, as when I left the San Andreas fault zone, I will be more than happy to experience a complete lack of catastrophe post-Brexit, and give all my supplies to the nearest food bank. But I do think there's a realistic potential for things to get difficult, and IMO that's worth a small amount of prepping.