>>125755you don't have to coop for story quests at all (the ai is very bad at chapter end boss fights though) but the event missions are all good for whatever they are labelled for
experience, money, character/building upgrades, etc
when all players can turn into dragons it makes things go by a lot faster
and when they can break shields because the way you touched the screen wasn't perfect so you did a roll instead of a charged attack
you also get gacha currency when playing with people but only till you get around 3000 and then it stops
coop has a separate stamina meter thing for those wings, where it will consume wings in order to clear a room someone else is hosting (host's stamina is consumed, not yours)
the wings regen to 6 but they cap at 50 so try to make sure you have 44 or under when you level up
each stamina point required for a quest, regardless of if you use stamina or wings to actually do the quest, provides 10 player exp, if you want to go nerd mode planning for efficiency's sake 
