If you regularly clear your cookies they will want to know who you are, as suggested above, so they can sell you more, track your purchases and activity and adjust the prices accordingly. Signing in lets them know that it is you and not me and charge you a higher price than me because they think that they know that you live in a very rich location compared to Brexit Britain.
It is very likely that two people in different locations with different online records will be charged different prices for the same item on the same website at the exact same time. They want to know who you are so as they can adjust the price accordingly. I've seen good deals for products where the price changed as soon as I signed in.