Why would I want to use a bit coin?
Payment processors charge 2~5% on every transaction. Crypto currency fees are optional, and much lower if you choose to pay the fee.
Payments are instant. Bank transfers can take several days.
You don't get payments blocked by a bank that wants to tell you what you can and cannot do with your own money.
You can pay anonymously.
Your money cannot be stolen from you through a bank bail-in as was done in Cyprus and as is now law in places like Canada.
Bitcoins are limited in supply so they are deflationary (increase in value - gain purchasing power) whereas fiat is unlimited and inflationary (you lose purchasing power over time).
You can send money to anyone, anywhere in the world with no restrictions on how much you send.
If you send more than $10,000 to anyone or to yourself across "borders", you don't have a bank tattling on you to a government that will label you a terrorist or drug dealer or put you on a watch list.
You can take your bitcoins with you when you fly and have as much as you want and there's nothing that airport security can do about it.
Bitcoins put YOU in charge of your money. Banks and governments can't force you to do anything you don't want to. e.g. You can buy anything you want or take your money anywhere you want or send it to whoever you want and they can't stop you.
You'd want to use bitcoins because they give you some freedom.
There are more reasons.
The real question is, "When you have the option to use bitcoins (or any decentralised, open source crypto currency), why would you ever want to use fiat?"
There is no good argument for using fiat over bitcoins. All the problems/objections have been solved. I've not read 1 single good or valid objection yet. And I've read a truckload on this.