I didn’t know what AndyRM has said but it sounds right to me.
Data is a major asset these days to help retailers run a sound business so some companies will want to capture your details a) so they can recontact you with future offers b) get a better understanding of their own customer base and who their best customers are..
Think about how Amazon makes recommendations to you…
Back in the day, your local greengrocer did this themselves ‘Good morning MR Brooess, will you be having your usual today?’ etc but with ecommerce it has to be automated.
Other retailers recognise that forcing you to log in loses the sale and they’ll give you a login-free route to purchase…
Basically, like most business, sometimes it’s done well, sometimes not… capturing data to give you better service is good. Capturing data to spam you is stupid.