Maybe it's worth if for them, given that it is all they do they want to amke it as east as possible - (somewhere like Tesco or B&Q it's a bit of sideline to reserve instore stock ISTM), maybe people do it less for argos, maybe it's for another reason etc.
eg. Screwfix don't offer the service. You have to pay upfront.
Can you actually reserve/buy anything in Tesco store now anyway (other than the grocery option)? It seems to be the click and collect option for the online store.