The ban is really for lineup purposes. Since in a lot of pro events you know the bracket ahead of time and can do research on your opponents' usual decks and play styles you can tailor a lineup for your group stage. I'd assume most players are like "Make it out of groups and see what happens after that." because of the variance involved.
You have to ban a deck. Iirc in the finals of HCT each player actually adds their final deck to their pool and you have the best of seven. The ban means you can construct lineups that are weak versus a popular deck. Like, let's say that Mechathun Warlock happened to be the most popular deck, you could ban that and only bring control decks. Obviously, if you happened to run into an all mechathun lineup you'd be pretty fucked, but that's something you account for in your research.
The other strategy is to just bring the strognest decks available you are comfortable with and ban your opponent's strongest deck. Last format this would have most likely been hunter. It's also pretty common to see players ban a deck they have not seen just because they don't want to deal with it, but these weird decks don't often seen play at very high end tournaments.
The ban system is also why tournament decks differ so much from ladder decks in terms of the card choices. Being able to elimiate a threat and know that your opponent is probably going to play certain cards allows you to tech your deck much more effectively.
But if it's conquest rather than last hero standing, why the hell do I care about banning at all? How does banning a deck add any strategy beyond "you can't play the #1 best deck now, so you'll have to play the #4 best deck instead"? ---
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