In my opinion, the biggest advantage is that it doesn’t have all the extra (and useless) parts for DA.
Just one part, I think -- the disconnecter -- is dropped on the SA (only) model; internally, they are otherwise the same.
The SA model has some features not present on the DA/SA models: 1) two-way adjustable trigger (which can't be used with the DA/SA models); 2) larger and ambidextrous safeties; 3) a different, extended beavertail, and 4) the slides have extended rails, but I'm not sure they are fully functional. The 40 SA model (along with the .40 DA/DA model) has a full-length guide rod.
A DA/SA model can be easily tuned to have a very good SA trigger, but the dedicated SA model is considered asier to shoot quickly and accurately.