Im not sure why that’s so strange to be honest.
Argos have a much bigger range, higher average item value, higher average item cube, none of it short life, so stockholding and the amount of cash tied up in stock is a massive issue for them. Concentrating stock in a smaller number of locations means they can - as perverse as it may sound - offer better availability and reduced stockholding. There are also the obvious savings from closed stores. From a customer perspective, people don’t browse products in Argos, to all intents and purposes it’s a pseudo-Amazon so “dark” stores and click and collect / home delivery is logical, and Sainsbury’s stores with space carved out for an Argos c&c is perfect.
For Sainsbury’s it’s the other way round. They have the store estate, they have the supply chain infrastructure, and can sweat the asset a bit more by picking home delivery orders from the main store. They can also use cross-trained staff to work both conventional shelf filling and delivery picking. It also helps massively in reducing markdown and waste on fresh, which costs millions each week across the estate.
The only downside to picking online orders from a trading store is that you tend to get a higher substitution rate where items were available at the point of order but not by the time the order was picked, because no stock system can have visibility of what’s in a customer’s basket at any point in time. There’s also more damage in a store and more theft. In a fulfilment centre stock accuracy is much better, and the customer facing ordering system is integrated into the warehouse management system so there’s a constantly updating view of what’s physically there, what of that is assigned to previous orders and what’s therefore left for new orders.