The price of Bitcoin is determined by people's needs. Daily price fluctuations are due to someone buying and selling Bitcoin. The high volatility of Bitcoin indicates that the daily trading volume of Bitcoin is also very large.
The purpose of our investment in Bitcoin is to profit from it, and the profit is to satisfy our daily lives and improve our quality of life, so why not use Bitcoin after we invest? Investment is not just to make profit after accumulation, but to make this investment meaningful.
The price isn't a function of need at all, but overall supply and demand at any given time. Since bitcoin isn't actually needed for anything in daily life though and the only people buying it are speculating on price, demand is a function of speculation more than anything else. Bitcoin is purchased exclusively by people who have money they can spend on speculating the price will go up, since obviously people who can't afford it won't be able to.