I play a range of games on my phone at any given time. I addition to this one I’ve been playing “Space Justice”. I’m sure it’s not the only game that does this, but it offers a special currency for players who aid other players with tasks. One thing Ive noticed is there is actually competition to help others as a result - you can’t click fast enough to join in half the time.
I think offering coins for this activity in this game would some some issues. Tasks like the new boss battles would have an incentive for other players, and folks who otherwise are trying to grind to another upgrade would have a needed additional source of coins to farm. Doesn’t have to be much per assist, but it would be available.