> could a cryptocurrency be used as a normal currency?
They mostly aren't suited to this. A desirable property in a normal currency is relative stability (generally with small, managed levels of inflation to encourage useful investment). But in order to bootstrap a cryptocurrency you need to get people interested in using it to establish a value, so you set up the currency in a way that disproportionately rewards early adopters and encourages speculation, to pull people in. This in turn makes it a lot less stable and you see where this is going
> What would be the benefit of using a cryptocurrency?
For those who are true-believers, the idea is that you have currency which is free from government manipulation, free from authority, is unstoppable, uncensorable, irreversible, may be anonymous etc etc.
For those of us who are not, most of those are actually negatives. Plus an honest evaluation of the largest cryptocurrencies often shows they don't fulfil these pipedreams anyway. Bitcoin, for instance, can be censored if wallet addresses are known and the government manages to get 'miners' onboard with a blacklist. Exchanges can refuse to accept deposited funds unless they come from known, KYC/AML compliant organisations. And given that mining BTC is a huge, expensive, energy and equipment intensive operation, there are not actually all that many firms, so collusion is a definite possibility, and with about half the hash power they could collude to block transactions.
> Is the goal to be used at a large scale globally or would it at best remain niche?
There are as many aims as there are users. Some want complete freedom from government monetary interference, implying large scale use. Some want to revolutionise the finance sector and disenfranchise the established banks. Some are basically addicted to gambling. Some want to get rich, and see it as an easy way, or the only way. Some just want to facilitate 'dark' transactions (contraband of various forms).
> Does it has to be stable?
No, but there are so-called stablecoins which attempt to peg their value to normal currencies, specifically the US dollar. Some attempt to do this algorithmically - see Terra/Luna/UST and the chaos from that collapse earlier in the year. Most algortihmic stable coins are vulnerable to some sort of bank-run scenario.
Others attempt to do it by having their stablecoins backed 1:1 with the normal currency they are pegged to, the largest of these is "Tether" also known as USDT. They used to claim there was a dollar in a bank-account for every token issued, but they refused audit for years and eventually dropped the claim. They have various redemption barriers ($100k minimum payout, for example) to stop any sort of run from happening, and there are strong feelings in crypto communities and sceptic communities that they are playing fast and loose, and have perhaps backed a lot of their tokens retrospectively (create token, use it to buy an asset, now that token is backed) or purely manipulatively (giving out billions to friendly exchanges to use to prop up BTC prices without getting any more backing than an IOU).
> Does it have to follow a real currency like the USD?
That seems to be the gold standard here! Though stablecoins against other currencies probably exist, and I believe I heard about gold and silver backed stablecoins at some point. The problem with all of the backed ones is "Hey, I have this massive bank account, just sitting there backing my coin, surely nobody would notice if <shenanigans>"