Bitcoin started as way for everyone to send money without needing banks or other companies. But because its price goes up and down much people now mostly see it as something to save money in hoping it will be worth more later and it has made money for many people who have held it.
Price going up and down is not a complete reason. I don't think anyone would invest in something that fluctuates(going up/down and returning back to it's normal state), without any attributes of a better growth. imo, the best reason was that, Bitcoin can increase in a long term even while volatility still does it's thing(up and down movement). I was trying to understand it's price growth mathematical model, and I actually discover that it doesn't follow a complete exponential pattern since it's price doesn't necessarily double after a complete cycle. I think the "power law" behavior describes its price movement better, since it's priced increases over a long term even with the influence of volatility.
Having steady job and income is most important thing before even thinking about putting money into something as unpredictable as Bitcoin. If you do not have regular income you will be very tempted to sell your Bitcoin if you have any money problems and that mostly leads to losing money. And idea of buying Bitcoin regularly with DCA is good way for many people because it helps reduces risk of buying at wrong time and encourages thinking about it as long term investment.
Having a source of income is not even negotiable. And not just a regular source of income, having a good one also gives you a better chance of accumulating more fractions, setting aside your emergency funds(if you probably haven't done that from day one), and also setting aside your basic needs as well.