"Only invest an amount you can afford to lose."
Honestly, this is still what I keep until right now,
I always use money that I will not use for daily needs or other needs, I mean maybe free money to invest. Additionally, I am also always investing money only in the amount that I can afford.
Although that kind of finance and other fiat problems may happen, I am still not ready yet if I should lose money more than I can afford. So, investing wisely is better for me. AT least, I have enough investment but without over risking my money and my life.
But, I know that everyone also has different opinions and considerations. As long as you can afford it and are ready for whatever will happen with the investment, it may not be a matter.
For me, I think that part of Investing funds that I am not in need of is a better way to avoid panic. Even when the market goes down, you are really not in need of the funds so you can wait it out but for those investing the only little amount they have hoping to make a quick profit, then I see it the other way round, panic over every little dip because they are afraid to lose that investment. So you are more relaxed investing your free funds IMO.