Not only fiat. You need them also to convert it into any other crypto.
Its like a list of all the small shop money exchanges that exists. But it doesn't mean its decentralized. It also means you always need a trusted third party for the exchange.
Wasn't crypto about going rid of those trusted third parties?
For most cases, yes. Binance, Gemini and some others provide a version of their token that can run in multiple chains. It's supposed to be 1:1 backed by BTC, but you have to trust them fully.
It's a little bit better for WBTC [0]. WBTC is a consortium of 30+ crypto projects who keep the BTC under their control through a multi-sig wallet, and that can be audited on-chain. In this case, the majority of the signers would have to conspire in order to steal the funds or do anything malicious.
renBTC is even more decentralized. It is part of the ren "network" [1], which provides inter-blockchain applications. I haven't looked yet how it works exactly, but my understanding is custody of BTC happens though a proper dapp. Last year I wanted to get rid of all my BTC, and I did it though ren. It was BTC -> renBTC -> DAI.
[0]: https://wbtc.network