Sophie,
Mainly to get rid of the smell. It is possible that the jet would fail other quality control tests (Doctor test, Copper Strip, JFTOT) if the mercaptan sulphur was not treated but I'm not sure as it is usually only fully tested post mercaptan treatment.
Here is a copy of the specification:
http://www.dstan.mod.uk/data/91/091/00000600.pdf
Peter