I have to admit that when people speak of "digital" currency, I have never had explained to me what that means and why it is, supposedly, a good thing.
A government issued digital currency isn't necessarily a good thing. When there is a USDC (official U.S. digital coin) backed 1-1 to the U.S. dollar that would be a "stable coin" but it would mean that the government would always know exactly how much money you have and what you are spending it on.
Most people who favor Bitcoin don't want a digital dollar. By the way that "stable coin" is only as stable as the dollar (which isn't very stable).
As for paying for a hot dog, or a newspaper, with bitcoin, I bet you can't in practice. So it's pretty hopeless as a substitute for the national currency in your (analogue??) bank account.
You can do it easily enough where that's what merchant and customer want to do. You can also do it in a country where you don't trust the local currency. Some currencies, due to inflation, are soon virtually worthless so you convert it to Bitcoin right away if you are working there and being paid in the local currency.
There is also the benefit of the blockchain as opposed to the currency use. There is the Lightning Network where you can use the blockchain seamlessly without even having to be aware that it's using Bitcoin.
You have the Lightning Network app on your phone, buy something priced in dollars, the merchants barcode reader swipes your phone and they get paid immediately with cleared funds. The LN app, in the background, converts the dollar price to Bitcoin, goes to your bank dollar bank account and pulls out that amount in dollars and converts to Bitcoin and Bitcoin travels on the Bitcoin blockchain.
Both customer and merchant have dollar accounts but the amount moved was Bitcoin on the Bitcoin blockchain. It's instantaneous and funds are cleared from the start.
You could hop on a plane and buy items from country to country and never have to deal with "cleared funds" or any time delays. In theory, you could live in Russia, hear a knock on your door in the middle of the night, grab your Ledger, sneak out the backdoor and hop on a plane to the U.S. with only the clothes on your back and your Ledger and when you got to the U.S. you could buy a house with cleared funds immediately (if you had all of your assets in Bitcoin in self-custody).
That's why, in some ways Bitcoin is better than stocks, bonds, and property. If you owned property in Russia and had to get out, you can't take that land that you owned with you. You also have to constantly pay property tax. Stocks and bonds have issuer risk, business risk and risk from whatever institution is holding your ownership rights. Schwab could go out of business, Amazon could default and the stock would be worthless, the government could suddenly devalue all government currency. Bitcoin avoids all that.
Of course the dollar would be the last currency to have to worry about along with the Euro, Pound, Yuan. Eventually there will probably just be those but look at how many countries' currency isn't very strong. Even with the dollar though, governments do what they always do which is devalue the currency.
If you held $20 for 100 years it would be virtually worthless. An ounce of gold would also still buy you " a fine man's suit" whether that was in 1900 or 2022. The dollar value of an ounce of gold in 1900 would have bought you a suit then but now it would buy you a fast food meal.
Anyway, it's an interesting subject.
As you say, it's an investment commodity like gold, but more speculative. Many of us prefer to cover ourselves against inflation by investing spare funds in some kind of asset portfolio (stocks, property, bonds), with a more reliable performance record.
Bitcoin has only gone up over a relatively short time period. When measured against the dollar it is certainly volatile over the short-term if you are looking at it as a currency. If you are using it as a currency you just keep a small amount for that purpose and the rest is an investment. That's what you do with cash as well since inflation eats away at cash.
The stock market has gone down this last year, so has Bitcoin. It's not that dissimilar. We know to just hold and forget about stock prices when there is a down market. It's the same with Bitcoin. You just have more control over Bitcoin with self-custody.