My sibling and best mate both spent time in SS's as part of their childhoods. Both came away with real life skills and qualifications (yup genuine employer recognised bits of paper!!!) that they wouldn't have had a hope in hell of getting coasting along in mainstream. Both have gone onto higher ed as adults and had "proper" jobs as opposed to a life on the dole (harsh this one, but true).
Expectations - oddly there are often far higher expectations of what it's possible for your child to achieve than when they are babysat in the bottom set in mainstream. Your child will be given the opportunity to achieve their true potential - at whatever level that may turn out to be.
Educated educators, I'm just so TIRED of explaining DS's difficulties to the unitiated. A safe environment free of the taunts that are so easily thrown in the direction of the SN child as they go further up the mainstream age range (teens esp can be just plain cruel). A sense of belonging rather than becoming increasingly isolated as the ability gap widens.
Quick access to specialists - many SS have SALT, OT etc onsite at least some of the week, every week rather than the one day per half term so often seen in mainstream. More support for you as a parent - no running the gauntlet of the judgeypant playground clique parental tuts the day after a meltdown.
One though - SS can vary in quality just as much as mainstream, in some LA's they risk becoming a "dumping ground" so do go see for yourself to ensure your choice is really a "centre of excellence" for your own child's specific issues iykwim. Not all the benefits above apply to every school, but the possibility is certainly there.