Because you're able to move the camera in real time, ingame. You see immediately how it looks like. Especially when your resolution varies from the default one in teameditor. Also you're not only able to edit the position, but the angle (fov) too.
Another thing thats completely off topic, how can I race on the singapore circuit because even though I put the files in (file in circuit folder and file in main gp4 directory) it still comes up with the USA circuit...