No mate, it's not pointless.
I think staff on the shop floor who aren't behind a perspex screen should be wearing a mask, partly for health reasons, but also to set a standard for their customers to follow.
But, to take Morrisona as an example, there's very roughly, about 200 staff per store, and again, very roughly , each store gets about 3000 customers per day.
In reality, a lot of those staff won't be customer facing, so it'll likely be between 100 and 150 per store.
Simple maths says if all customers wear masks, that's roughly 95% of the people who are in the store each day. Plainly the staff are in the store longer than the customers, but even so, based on 8 hour shifts and half hour shops, if customers wear masks and staff don't, then you're still making a difference by putting on your mask to do your weekly shop. But it would be better if everyone, including staff who weren't behind a screen or visor, wore masks.