Studio One, like most DAW's takes control of the DAC and assigns the DAC output to the master bus controller. In my case, and in the case of many small studio owners, the DAC output is routed to a pair of powered monitors, meaning the output of the speaker is now directly controlled by the DAW master bus slider. Having the master bus slider at a level required for mix is now a completely different thing that listening, not going deaf and not having the neighbors call the police. The answer to this to have a mix-down bus that is not connected to any hardware, then route a send from that bus to the master. The master then controls volume and mix-down comes off this new non hardware bus.
Further - having an ability to mix to a wave file from any bus opens up a whole world of alternative mix possibilities from one file. I frequently have alternative mixes to different buses, but in S1 I need to assign hardware to each bus that I want to export. If I run out of outputs - which is very easy to do if I'm traveling with only my laptop, then I can't do this.