Oh, you want reasons?
Galleries - new window so that if the surfer gets jerked to a trade, they will either close it on impulse, or surf around that TGP and eventually close it. Either way, your TGP is still there, ready for more potential clicks.
Perm trades - better productivity if it's in the same window.
Sponsors - if you open it in the same window, that's one less window with free porn in it to distract the surfer.