Doesn't make sense. It's like wearing a condom for the first few thrusts then taking it off - takes away the whole point of the protective measure, so you may as well have not used it at all.
COVID spreads exponentially. So where people catch it barely matters because they take it home to families etc. John can catch it in the pub, then take it to work and spread it to Bill and Jane, while little Jimmy goes to school and brings it back to mum and so on. Doesn't matter if they all wore masks at a supermarket or on the train, and it doesn't matter if everyone else did, because there's so many other areas and routes of transmission. So you either have protective measures or you don't when it comes to limiting transmissability. That's what a lockdown is for.
The only reasonable argument for supermarkets and travel alone is to protect the vulnerable, but again, the situation hasn't changed - there's 2 Omicron cases in the entire UK, so there's no sudden reason now for a mask mandate to protect anyone that didn't exist days, weeks and months ago.
It's a tokenistic measure. It serves no real purpose. It's being done to 'do something' rather than for any logical reason.