It's not that, trust me. They make good money with your data. Take it as a way of payment for the "free" wifi.
It helps the same purpose as the loyalty cards, especially the ones that outgrow the original business (I'm looking at you both Tesco ClubCard and Nectar Card). Getting "points" by using those at other businesses like petrol stations helps them profiling you for "better" advertising. They also keep you a bit more loyal to their associated brands, but we already knew that bit :)
I wonder when that better advertising would actually come along. They keep collecting the data but so far all the ads I've seen is either utterly irrelevant crap or "you visited shoe store so our network would show you the same shoe store's ads for the next 3 months, because it can't be that you don't need buy new shoes every day".
It helps the same purpose as the loyalty cards, especially the ones that outgrow the original business (I'm looking at you both Tesco ClubCard and Nectar Card). Getting "points" by using those at other businesses like petrol stations helps them profiling you for "better" advertising. They also keep you a bit more loyal to their associated brands, but we already knew that bit :)