They offer that information as criteria the ad campaigns can select for. The criteria could be pieces of specific datum or analyzed conclusions (e.g. predicted sexual orientation, political affiliation). Those conclusions may be inaccurate but they are still more accurate then trying to make those conclusions in a tv/print campaign. So it's still effective.

There may well be a company selling big-ass belt buckles with anime themes that is trying to reach you. When designing their ad campaign they could say they want people with an interest in anime/manga AND interest in westerns, trucks, boots or jeans. They would have no way to do that with a traditional media campaign.