Obviously the reason would be to increase the revenue for the tournament organizer, which would hopefully result in bigger tournaments and bigger prizepools. And yes, it does add to the "legitimacy" of the entire thing. I wouldn't ask anyone to be happy with advertisements, which are obviously annoying - I'm just asking if they'll be legitimately angry, if they had to see adverts during professional games, placed on the maps.
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The advertisements would only be displayed in streamed professional games.
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The advertisements would only be visible to spectators/viewers, not to players.
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The advertisements would be placed above the actual play area (such as overhangs/top of doors), as to not interfere with the viewing experience.