Cryptocurrency can possibly be seen as a commodity, a form of digital gold (in the case cryptos like Ethereum, "digital oil"), stocks and shares in technology, and so on. But why would anyone want to use something as a currency that could potentially go up 2x, 10x, 20x in a few months?
Normal fiat currency like USD and EUR is relatively stable and lacks volatility, e.g. the price of a can of Coke hasn't changed much in 10 years.
Crypto is completely different. it's far more volatile and acts like stocks/shares or investments rather than national currencies
If I have holdings of fiat and holdings of shares - which should I use to buy a TV now? the fiat obviously
Why liquidize an investment that could be worth far more in the future over using fiat that will probably be worth approx the same?
For the past 10 years inflation has been exceptionally low in countries such as the US value of the fiat currency hasn't changed much in value. On the hand, in countries such as Argentina with hyperinflation the value of fiat currency has changed dramatically.
And yes, some cryptocurrency are designed to be held as an investment. After their ICO few rise or fall in value of over 100% in a few months. For certain, bitcoin did between November 2017 and February 2018. Since then, it has been stable. The past month bitcoin has been more stable than the USD.
Lastly, people sell
some their investments (stocks, bonds, commodities, precious metal, and so forth) all the time to get fiat currency to purchase goods and services including TVs.
What I think impacts the decision to use cryptocurrency for a medium of payment is what benefit does it provide buyers and sellers. Consumers that don't have access to banking services such as bank accounts or credit/debit cards it provides a way to make electronic payments or remittances anywhere in the world. For sellers it provides an electronic (not cash) payment with little to no transaction fees.