There's ancient history, and there is modern history.
Originally that track allowed yard movements between Bathurst Street (when it was a freight yard) and the Inglis plant and the considerable yard and industrial trackage west of Strachan in the Liberty Village area. In those days the main line was controlled by towers (Cabins D and E as well as CP Tecumseth St) and the "Western Lead" just let yard traffic run back and forth without getting in the way of movements at Cabin D. Trains departing Bathurst Street Yard could join the Oakville main line at Cabin E
The more recent history is that when the Bathurst Street junction was rebuilt with the flyover and a GO layover yard, and Cabin E deactivated, the lead was retained as it was the only direct route between the GO layover yard and the Oakville Sub - a convenience for GO equipment moves. There have since been changes in the trackage leading to the Weston and Galt Subs that took some of that connectivity away, but by then the track was wired into the signalling so it was easier to leave it there than to tear it out.
The track has been in and out of service in the past few years. I was always surprised to see it used, mostly at rush hour when things weren't going smoothly and there was a backlog of equipment moves needing to reach Willowbrook.
I have to admit, it's a most improbable bit of design that seems to have hung in there as a workaround to the overall design..
- Paul