If you traded for USD to BTC to USD under normal circumstances, you've got:
1. Alice wants to buy something from Bob.
2. Alice buys BTC for USD, incurring one TX fee.
3. Alice pays BTC / mining costs to transfer BTC to Bob, a 2nd TX Fee.
4. Bob transfers BTC to an exchange for USD, a 3rd Tx fee.
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Now lets say everything were centralized.
1. Alice opens an account at FTX. She buys "virtual BTC" (just an account at FTX, not a real wallet transaction).
2. Alice transfers money to Bob, possibly through FTX.
3. Bob sells this "virtual BTC" back for dollars.
4. Note that unlike the first example, this transaction is instantaneous. The first example requires waiting for confirmation at each transaction. Confirmations nominally happen every 10 minutes, but there have been times when the wait has been days (remember when Steam adopted BTC and the network couldn't handle the load? I remember)
FTX can charge a transaction fee at #1 and #3, and still the whole process will be cheaper than the 3x transaction fees incurred over decentralized methodologies. Of course, when FTX collapses, everybody's money is gone.