Presumably it saves on staffing costs too.
I'm not sure about returns. There's the wardrobing trend we hear about, where people order clothes, wear them once or twice, claim they didn't fit and send them back. Some people, so I've read, even have special label attachment guns at home for this purpose. I'm sure there's at least a bit of exaggeration there, and it probably applies more to the online, clothing-focused retailers, rather than Matalan (who I would have associated with cushions and Ikea-type stuff, but I'm pretty sure I've never been to one).