In some countries value held in cryptocurrencies is legally considered to be an investment. For example, were I live there is no capital gains tax on investments, so if I earn large sums of money speculating with cryptocurrencies then I earn this tax free. But if I gamble on some website and win the same amount of money, then I have to pay a tax over my earnings.

Cryptocurrencies do have a practical use as a means to transfer money. While PayPal is cheaper to use and has other benefits, as pointed out in the answer by NoAnswer, the sanctions imposed by the US on Iran makes it impossible for many Iranians to get paid using PayPal or any of the other conventional means. For example, Iranians are banned on many online platforms where they used to earn money in exchange for work, like writing software. The solution to this problem is to use a middleman who receives the work from the Iranian, submits it and gets paid. The middleman then transfers the payment minus a commission to the Iranian using Bitcoin.

The use of such middlemen is legal in most countries, even if it is used on US platforms, as the sanctions imposed by the US on Iran are considered to be illegal in most countries. So, this is a legally allowed use of Bitcoin which creates economic value by exploiting the US sanctions and Iran's workforce.