[furi]

I can't speak for the others but in the case of Dota 2 the "true" subreddit just discusses the strategy of the game itself while the main one posts memes and fan art and talks about meta topics like eSports and the microtransactions.

[frickenhamster]

meta topics are what most people care about. "true dota" is a bunch of 3k players trying to give generic advice.