What is the difference between why are you coming? and why did you come? and what brings you here? ?Feel free to just provide example sentences.

Why are you coming ?
He is still on his way to the place , for example you knew that a guy you don’t like is coming to the party and he’s with you on the phone * phone call * , so you say “why are you coming to the party?” The party here hasn’t finished yet , and that guy didn’t arrive to the party yet .
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Why did you come ?
Same example, let’s imagine that same guy has arrived to the party , you don’t like him , you don’t like him being in the party, so you tell him “ why did you come?” The party is still on , but your friend has arrived .
But it could be used as well if the party finish , maybe you could ask him after the party “ why did you come?”
Here the party finished and your friend arrived, it could be used weather the party is on or finished, but your friend has to be in the party or went to the party.
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Why did you come = what brings you here