Because Fullscreen windowed looks identical to Fullscreen, except that your fullscreen gamma setting will be grayed out and not applied, and any windows that are 'always on top' will be on top of SWTOR during play.
While the image quality MAY appear the same, as a person who likes to seriously tweak their games for maximum performance, there is a HUGE difference between running in Fullscreen(Windowed) mode compared to dedicated Fullscreen. When in windowed mode, the windows UI is still mostly dominate and the game is being drawn inside of it. The dedicated Fullscreen mode bypasses the windows UI and takes over the screen to allow for maximum performance.
Even with a top of the line gaming rig, I still refuse to run my games in windowed mode. On some games it only gives me a 1-2fps increase, but I'll take anything I can get.