It's a bonus round because you are not expecting to win it because your team bought up in the second round after winning the first, you have lesser weaponry than the opponents full buy but if you win its a "bonus"
The round after anti-eco, in which you play the round with the smgs you saved, not necessarily a round you need to win, hence, bonus round. It also helps if you win because you'll be building economy when the opposing team is, again, need to do an eco round.
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