To "safely" jettison a missile it needs to go in an inert state.
You can lob a 'winder of a rail as only the (seperate) motor fire line is energised. Whoosh....
Same with 'flash/sparrow, as has been said, energise the eru carts and the rams throw it away inert.
If 'flash or sparrow were on a rail you still have seperate motor fire lines to, this time, let it help itself away, but remembering you now have an inert, very explosive, high speed, sub telegraph pole stooging about the atmosphere.
Unfortunately I will have to stop here, any further comment on more "modern" equipment would be ill advised....but I think you get the jist...