I’ve wondered this myself. My first inclination has always been to concede when defeat becomes inevitable, which feels perfectly natural in a game like chess. But in the culture of online game-playing concession seems to be taken to express poutiness. Whatever the convention I’m happy to follow it, although some of the alternatives offered (e.g. suiciding the summoner) don’t seem any better.
What does baffle me are those inexplicable concessions. Like, the other guy’s trouncing me, my units are obliterated, my summoner has two freezes on him, Krung’s going to be in battering range next turn … and then all of a sudden he concedes. Who knows why?
(On at least one occasion in another card game, the other player brought me within inches of death — literally nothing I could do to change things — and then just held off. Did nothing during his turns, refusing to make the final blow. The only thing I could come up with is he wanted to force me to concede; at that point I kept it going just out of spite. It took something like fifteen minutes before he finally finished me off.)