Some people such as Sarah do abuse them being open and go shopping just for fun. But then, some people just go in Asda for the non essential sections too. Products are often cheaper in the discount shops and some people can't afford to pay more for the same products even for a few months of lockdown. Even if the pack sizes are smaller and it would in the long run work out cheaper to buy bigger sizes in the supermarket, some people just don't have that money all at once. Where I live they are also very conveniently located so for some people are much easier to access for their essentials than having to go to a supermarket, which they may not be able to get to or very small local shops that charge a fortune for a pint of milk. It would also be a nightmare to get in the supermarkets to do the shop if they were the only places open, having more open does help stop crowds being in one place at any one time.
There are some things that I can only buy at home bargains, but I currently go every few weeks and stock up because I can afford to do it all at once - but then at the same time have to be conscious not to take too much!
Sarah is almost certainly going there for non essential reasons, they don't really clean or use toiletries, and I feel the pain meds Chris wants are beyond what they will have there, so there can't be many essentials for her to buy!
The biggest problem is them going shopping almost daily when they are clearly in a position to do a bigger weekly shop. They should have the money, and if they don't have the fridge/freezer capacity then that's really their own failing and something easily sorted out if you can afford it.