Cryptocurrencies have an inherent utility as a technology. People also buy them to use them as a monetary asset, and demand and supply determine their value. Now this is similar to buying land – you can buy land to use it (build a house etc) or invest in it. Likewise, people invest in cryptocurrencies. These investments are used by cryptocurrency developers to fund further development of their projects. Given the tremendous potential of this technology, investments here will reap high returns. Taking a note of this companies such as Tesla are investing their treasury reserves in to bitcoin.
Interestingly, cryptocurrencies have delivered the highest returns on investments in a ten-year period than any other asset class in a lifetime. This is also to say that a critical mass of investors believe in the potential of the underlying technology.
Yes, cryptocurrencies can be volatile and open to speculative trades. There have been reports of frauds and market manipulation. However, such activity can be regulated through licensed intermediaries. For instance, certain countries only allow sophisticated investors (having a threshold net worth) to invest in cryptocurrencies.