I don't know what's the reason behind why Qatar wanted to host World Cup because it's probably not the money they are after because they are already petrol nation which economic doesn't need some more boost as with or without World Cup, their nation is already rich enough. And I doubt if they after the tourist because from my perspective, they aren't much of a tourist friendly nation.
When you have endless money, then you want something intangible like fame, world recognition, etc. Otherwise, why are these sheikhs competing in crazy projects (including buying up European football clubs)? It was something from the same area. It's not economics, it's closer to politics where rational considerations don't work.
It is not only politics, or at least it is just a part of all of this, countries which are rich on oil and which depend on it to sustain their economies know very well that one day such bonanza is going to run out, so they need another source of income for their countries and they cannot wait until that day comes as it would be late at that point, so they are trying to build their countries as touristic attractions and for that they need to spread the awareness of their countries, and Qatar found on the world cup a way to do this, after all there have been studies made which demonstrate many people cannot even find their own country on a map, then how we can expect those same people to know where Qatar is unless they did something extraordinary like hosting the world cup?