From what I heard, they were heavy contraptions that pretty much did the same thing as your standard tube-fed pump or semi auto scattergun, but looked cool (or ugly depending on your point of view). There was its successor, the SPAS 15 which was similar, but mag-fed rather than tube fed, and sadly, also caught up in the gun control hysteria that was reaching its peak during the early '90s.
The only thing special about them, was as mentioned, they had the ability to switch between pump and semi-auto only operation.
There were some Benelli scatterguns (Namely the M1 and M3 tactical models) that were also banned with the SPAS, possibly because, to the clueless gun grabbers, the pump/semi-auto select switch sounded too close to a "Select fire switch". They had to backpeddle a bit to exempt a number of hunting shotguns which used the same basic action as the M1 and M3 series, and some mag-fed versions also got through as well...