The main point of Tethers is to be able to transfer value in USD without currency risk or using the banking system (speed and cost). I believe many use them to get USD onto exchanges that they cannot directly deposit at. Also, the exchanges buy them for their customers when they want to withdraw to another exchange. I imagine a large amount of the USD margin lending came into the system via Tether
So basically those depositing USD to get Tethers created would be people that for some reason can’t buy in traditional exchanges like Bitstamp, Kraken, GDAX, etc. So it must be either people that want to invest a large amount of money at once (which isn’t possible on those exchanges because of low withdrawal limits), that want to buy cryptocurrency fast (assuming the process to get Tethers is faster than being verified in exchanges) or that their money ins’t accepted for some reason but Tether does accept it (because of being located in a country with a laxer regulation)