In some countries value held in cryptocurrencies is legally considered to be an investment. For example, where 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 has economic value by exploiting the US sanctions and Iran's workforce.
The enormous volatility of cryptocurrencies attracts people who are into online gambling. This fact has been used an an argument against using cryptocurrencies as investments. However, there fact that there exists a large market for online gambling, means that the average value of the popular cryptocurrencies like Bitcoin is unlikely to collapse to zero. Investing in Bitcoin when the price drops sharply therefore does make sense.
But unless investing in cryptocurrencies is going to be more rewarding than investing in other assets, it's not going to be seen as a proper investment. So, are there a good reasons to choose to invest part of your portfolio in cryptocurrencies?
The enormous volatility due to the limited practical uses attracting mainly gamblers, makes it useful to invest a fraction of your portfolio in the popular cryptocurrencies like Bitcoin and Etherium. The optimal profile of a portfolio is such that a fraction is invested in extremely risky assets that have a large growth potential, but can just as well collapse in value.
For example, if your portfolio is worth $1 million, then investing a few thousand dollars in Bitcoin is a good choice. Should Bitcoin go down sharply in value, you would only lose a few tenths of a percent of the value of your portfolio. But while the value held in Bitcoins can only go down by a fixed amount, it can go up by an unlimited amount, the large volatility makes such upward fluctuations more likely to happen compared to more traditional assets.