The reason is simple, the main line is basically heading North - South, the line to Parkes/Broken Hill runs in more a Westerly direction at that point for down trains but for the many up trains they came into the junction fork in an Easterly direction.
There are many reasons why it was constructed but, the best reason was that most trains from Dubbo and from Parkes ran into East Fork and if they were block loads such as Wheat and the ore train W44 from Broken Hill an egine change was carried out there on the East fork of the Junction. Mixed loads that were either split or needed to be built up took the fork and went down into Orange Yard. The same applied for West bound trains, to both Parkes and Dubbo, although most Dubbo trains went via the main line for down services but up services in the main were via Molong and came into the junction fork
In years past Orange was a very busy location especially the yard area, along with stock yards along the area next to the up main near the loco depot that was perfectly placed inside the triangle.
In 1970, when the IP began running a station was built on the fork so passengers from Oge would board the train there, included in the ticket price was the taxi fare to that station, also passengers could alight there on the up services.
To go down to OGE itself for engines to run round their train and head back past the fork would take at least 1/2 an hour and that is with everything going your way, the speed around the north fork (my call on that) for a train to head down to the main station, or loop to run round is slow IIRC the speed was aroun 25KM/H while on the triangle lines owing to the point work each end. Then to run down at yard speed by the main and across to the loop, engine cut off, and run round, then whatever brake tests and coupling up etc needed before it could depart south again.
If a direct train to/from Parkes ran, the crews today would be running at least to Bathurst, so nil stopping at East fork at all.
There are quite a few sensible reasons for it being in use and why it even was placed there.