The trading feature, once that gets introduced (at least, I assume the sharing/trading will still be possible in SSF? Just can't share it across your other chars)
Playing with friends
Feeling less lonely
I think you might be conflating the announced CoF from the item factions announcement with SSF.
SSF is strictly a solo experience. You can't be in the same zone as another player, let alone trade with them.
CoF does cater to players who play SSF but doesn't restrict you from being in a party or using the gifting system.