But bitcoin is not just a currency but also an asset. Bitcoin is a commodity. Bitcoin is subject to volatile. The price of bitcoin increase over time and may later decrease. Bitcoin has been a hedge against inflation in long term. Some people and organizations like Microstrategy and retail investors are holding it and expect more adoption and more people to buy it for its amrketcap and its price to continue to increase just like gold. Because of this bitcoin is also an investment.
Bitcoin is volatile, the price moves against what? 1 sat = 1 sat. If the market decides a watch will cost 0.007 BTC then the price of the watch is 0.007 BTC. The problem starts to raise when you are trying to convert it to a fiat value. These days the concept we have for bitcoin has changed from the original concept, I think it's because of the industrialization of mining.