What is Special VFR? Why would a pilot request it?
Special VFR is used to leave or enter a control zone coming from or continuing the flight in airspace G (Golf), when the weather minima for VFR flight are not met within the control zone, but would be met in airspace G.

Example: Airspace G (Golf) requires a visibility of 1.5km and aircraft need to remain clear of clouds. Airspace D (Delta) requires a visibility of 5km and 1000ft vertical and 1.5km horizontal separation to clouds. 

If the weather report would show a visibility of 4km/4000m and a cloud base of 1400ft, the flight would be allowed in airspace G but not in airspace D.

Requesting Special VFR within a control zone will drop the VFR weather requirements to the values required for airspace G, so flight may enter or leave the controlzone. The lower visibility requirements are compensated for in the control zone by stricter separation requirements between aircraft. SVFR is separated to SVFR and IFR traffic.