And if your plan is to ultimately do the latter, you're going to have to cash out slowly and indirectly anyway, in which case why not pump the price of other assets you stand to benefit from in the mean time.
(2) They already had BTC and a belief in its future. Making it worth more, easier to exchange for cash and having more of it looks like a win all round if you don't mind committing fraud.
(3) Leverage, provided the BTC price holds up for long enough to cash out. Even if you're printing fake money, you can only print as much of it as people believe you actually have.
(4) BTC price rise income looks like it might be a legitimate income stream when auditors ask about it. Income from selling stuff that's supposed to be backed by a dollar for a dollar is obviously the proceeds of fraud.
(5) BTC holdings can likely continue to be bought and sold even if nobody wants USDT, bank accounts connected to Bitfinex and Tether end up frozen and a rapid change of address and identity is required
Money laundering is often very convoluted and usually costs money, but it's big business.
> “My personal biases make me inclined to believe that Bitfinex is not using Tether to manipulate the price of Bitcoin,” said Jeremy Gardner, a managing partner at the investment firm Ausum Ventures. “If they have attempted to do so, which would be deeply concerning, it’s hard to imagine that such fraudulent issuance at its current volume could single-handedly buoy the price of Bitcoin.”
My point is that being skeptical about their ability to manipulate the market in a meaningful way and make a profit doesn't mean they don't have a profitable motivation for engaging in fraud.