Most of the CWT members who use their real name as their username do so specifically because they are published authors and their posts are entirely related to their books or subjects covered by their books. In other words, their presence here is primarily or entirely for marketing purposes.
However, it is worth noting that one of the people here who posts on the forum almost entirely for marketing purposes is probably the largest publisher of Civil War books yet uses neither his real name nor the name of his company as his username (though the company logo does serve as his profile pic)
There are some folks who use first initial (sometimes also middle initial) + last name for a username, probably entirely out of habit from when they first started using the internet as that was how work, school, and ISP-provided email address were and are usually formatted. It's not an initial rejection of anonymity. I used to use that username format on other websites. The main reason I stopped is my name is uncommon so separated from an email address it's not obvious when reading it that it's a name or how it should be pronounced.
It's also a matter of branding. Just as an author or musician might use different pseudonyms for different genres, my real name serves as a professional "brand" as part of my day job. However, I'm here on CWT as a hobby, not to market my brand.