As of now, the main purpose of the bitcoin was an investment, since many people buy bitcoin because of playing the price in the market. many people expect the price will rise so that they will earn profit. In the future, additional common use of it will be as currency where they can use it to purchase something.
People are using it for investment but as bitcoin is a digital currency so its main fucntion is a currency but at this moment, it is only limited to people who are living on countries where it is legal and supported as a currency. Anither function is an asset or investment where investors and users are getting huge profit as its market value is getting mire expensive as time is passing by.