Special VFR is an authorisation for a pilot to fly within a Control Zone at night or when weather is below VMC minima although he or she is unable to comply with Instrument Flight Rules (IFR) procedures. Special VFR is designed to facilitate flights that would otherwise be restricted due to weather and the clearance may be given when traffic conditions allow.
I find the whole SVFR concept very confusing. I was under the impression that a basic PPL holder cannot fly SVFR in a flight visibility of less than 10km. And so the only purpose I can see for flying SVFR is to transit class A airspace. I'm probably totally wrong, if anyone can shed some light I would be very grateful!