I'm a proud owner of One Plus 6 and I have no idea why they have launched One Plus 6T. There are only a few differences in One Plus 6 and One Plus 6T.
The first and the main difference is the night mode. But after the launching of One Plus 6T this feature came in One Plus 6 also as an update.
The next difference is of the appearance. The notch and the fingerprint sensor. They have reduced the size of the notch and made it a dot and they have removed the fingerprint sensor from the back and put it on the screen.
Thus they have only made a mirror differences in the latest phone. So I don't see any idea of paying extra 3K for that . The latest phone has has extra color compared to the previous version of the phone( thunder purple) which might attract the customers.