They control the issuance of USDT, something they can't do with USD. They sure hope influencers don't immediately convert it to USD and buy other bitcoin or other cryptos with it instead. That way, they keep the USD and the influencer helps pumping the price.
Bitfinex could either issue some USDT, convert it to USD and hand that to the influencer, or issue some USDT and hand that to the influencer. Can't really see the difference here, except if they hand over USDT that will hype the market a bit as well? As long as the conversion works they two are functionally the same.