I don't believe there is a purpose for having it effect render output levels, but I do find it useful for monitoring which is why I use it.
Due to the way my interface ASIO drivers work, combined with the fact that my monitors take S/PDIF input, it's much more convenient for me to control the volume of FL's output is with the Master Volume control. At default, this sends a full-scaled signal to my active monitors which is Very Loud, so I have to turn it down considerably. This works out great, except when it comes to rendering. I often forget to default this control before going through with the render, making me have to do it more than once, and then there's that inevitable post-render "bloop" which ends up coming through Very Loud. I also have to be careful to remember to turn it down again later or risk waking the house up if I accidentally click on a kick sample or something.
I expect this would be a relatively simple change but it would be an appreciated usability boost for myself.
