I'm climbing the FIRE treadmill like this guy talks about. I live frugally, and I save like a madman. I'm on track to have stable, passive income for myself, and my family, for the rest of my life, by the time I hit 45. And I'm certainly not living like a monk along the way.
And you know what? I've got a huge war chest that I can pull from if I ever need to tell someone "Fuck you". I may not be financially independent yet, but I have more than enough money to tell a boss who wants me to do boring, shitty work to fuck off if I ever need to.
Thankfully, I don't have to right now. But things can change in the blink of an eye. Fuck you money is a condensed store of options for when things get rough.