Disclaimer: I'm not in Wales.
It strikes me as being rather a reverse of 'cutting off your nose to spite your face'.
I understand the rationale is to put, say, electrical good retailers who have to close on a level playing field with supermarkets. But I would have thought it's more important to keep the wider economy going as much as we can, while balancing risk.
As the supermarkets are open anyway, on that principle we should try to maximise what people are spending in them because it won't make any difference to the Covid risk.
And, as pps have said, some electrical goods will be more 'essential' than some grocery items. Things like, say, edible cake decorations will doubtless still be on sale in the baking aisle, but you won't be able to replace your kettle if it packs up.
I'm in a high risk area of England and hoping that if we go into a Wales-style lockdown, they'll still allow the supermarkets to sell a full range of items.