Why do you play dominions?

[LDiCesare]

The prevailing opinion was, play to win. and only play to win.

The question was not asked that way. You are inferring an answer to this question from a different question.
And to come back to the other forum question, your point was about ganking. Why gang? To win. If you expect people to do their utmost to win, they won't avoid it.

If I play SP, I clearly don't play to win the game. Never.
If I play MP, then I can play to win or to roleplay depending on the game. In both cases, the goal (roleplay or victory) is not what matters. What matters is how you get there. However, it's important in a MP game that people share the same goals. If you play an MP game to try that new "rush-my-neighbour-with-markatas" strategy, then you will likely have fun and lose. However, your targetted victim will likely not have fun and lose. Whereas if you both played to win, you'd share the same goals, and your neighbour would know what he can expect of you.
If you don't play for the same reason as other people in a MP game, you're simply running the risk of ruining the game for some of the participants.

is winning the only thing?

No, but in a game with no other factors defined early on, then it is the only goal you can expect from other players. The important thing is to know what everyone is after. In a competitive game on a forum dedicated to dedicated games, that's what you can expect.
Playing against other players who try to win mean you have to play better. It is fun to find ways of beating someone and also to lose to someone who managed better tricks/tactics/strategies.