It makes JSON deterministic, which it isn't by default (e.g. {"foo": 1, "bar": 2} and {"bar":2,"foo":1} are both valid serialisations.

Of course, it'd be better still to use a format _meant_ to provide human-readable canonical representations of data, e.g. Ron Rivest's canonical S-expressions (http://people.csail.mit.edu/rivest/Sexp.txt), but of course this is information technology and we have to reinvent the wheel — usually as an irregular polygon — every 3-4 years rather than using techniques which are tried and true.