Why do they need to see ID?

Shopping is far from my favorite activity and I had a highly questionable experience recently when exchanging a gift. As it happens, a brand name retailer wanted to see my ID for an exchange... why was this necessary? Legal identification has a great deal of personal information which a retailer has no need to access. Are they building a a marketing database? Are they planning to sell my information?

At this point, I will forego purchasing anything with this brand logo. They have enough on me; I see no reason to allow them more profit.

Note: I should have said I don't carry ID.