A lot of people online now have one or two usernames that are frequently used, be it for video games, online screen names for websites, or a variety of other reasons. Likewise people will sometimes use a false but consistent alias online to lessen how many places have their real name and weed out the real from the fake.
While it is true that this information, Usernames especially, can be used by multiple people, monitoring this information can help identify potential personal information leaks online that is not directly connected to the information currently provided.
For example, a games library with a collection worth hundreds or thousands of dollars would go by the username and not the person's real name. If information about it was stolen or being traded, it would be using that username, user number, or whatever other identifier that library has in order to trade information about it.
Likewise a person may use an alias or second name when signing up to a website. Therefore, if there is a data breach involving that service, it would be the alias that is leaked. This becomes a problem if the site required a card for purchasing a good or service as, while the card information has the person's legal name, it is attached to the alias and not the person's real name. This also applies if a website only requires a username. The username would be attached to the card information but not necessarily the person's real name.
