Sos battles do a bunch of things barely hinted in game:
! First and most annoying, sometimes another Mon will show up, like if you chain corsolas you get meranie, cubones to khangaskhan, and any baby in the first route to happiny, or if an evolved form shows up on the same route as the base in the dex, it will probably show up by chaining.
! Second and most useful: ev training, it doesn't just add the evs, some reports say that multiplies them, so be warry of a wingull chain and the speed stat.
! Third and most important: once you get to 40s hidden ability mons will show up, making it easier to access and harder to get.
! Fourth and kind of useless, just as 6th gen chains, you'll start getting better and better IVs as well, maxing at 4 perfect around 40 chain.
! Fifth and interesting shinny odds improve.
! A lot of mechanics were dumped in the sos battles, making them awkward, but exploitable way to get strong mons fast.
! The adrenaline orb item improves chances of happening
The target Mon being at low health helps.
It doesn't matter which one you keep or kill, but for capture purposes improving the chain you should always keep the latest alive.
Synchronize apparently still works, but you won't need to have a sincronizer for each personality, because the imprinted personality of the wild Mon will be the one of the front of the party, so you get a ko jolly rattata put it in the first slot, and then put on the second a synchronizer kadabra , half of the wild mons of the chain will be jolly.