why try to create a pegged instrument, why not just use the real instrument
It seems like a question any economist should be able to answer for themselves. People make futures contracts for oil, corn etc. because a derivative of a commodity doesn't have entirely the same characteristics as the base commodity. Just in the same way that buying a futures contract enables certain things that buying the commodity itself does not (especially hedging against unpredictable future events), a crypto derivative has characteristics that "real" fiat does not have.
Even custodially backed stablecoins like Tether (which I don't recommend anyone use), while it is not truly censorship resistant, it faces different censorship risks. If Tether claims that your money is stolen, then it is possible they could freeze your account as they have several times in the past. However, for example if you were using it to evade capital controls, it is pretty unlikely Tether would freeze your account. A bank account would not work for this.
I remember a couple years back a study was done on what the primary usage of TetherUSD was, and at the time it was mostly used for moving money across the border between China and Russia. Clearly the people doing this must have had a reason for not using bank accounts to do the same thing (presumably it's more difficult, or expensive, or perhaps they were doing something illegal and avoiding banks for that reason).