Given the staggering amount of men and materials that were moved by train in those dark and distant days (no forty ton lorries, no motorways) I rather suspect that it was one of a number of routes that were transporting stuff to the south coast via Redhill and Tonbridge.
The chord didn't go this way, but went from the Horsham line onto the Guildford line. It had been built, and appears on maps, from back in the 19th century, initially as a link, and by 1914 was a siding from the LBSC Horsham line, so was presumably owned by them. I can imagine it was usefully reinstated as a means to avoid Redhill, whose need for reversal in a confined space was a gross inconvenience in both wars - it is surprising a flyover was not slammed in.
Although the chord was this south-to-west link, it actually lay to the east of the Horsham line, joining the Guildford line just before its bridge under the London line. If you look at a current map, the Horsham line approaching Dorking makes a reverse curve to the west, while the chord had carried straight ahead, almost as if it had been built first and the London line was a later addition.