You don't need to say the literal words "fuck you"! It just means, you can quit your job without a new job lined up, because you have enough money to survive an extended period of job hunting. Or that you can speak up about issues in the workplace without fear of being fired, because you won't go bankrupt.
Yeah, absolutely!
A few years ago, I really unhappy at my job. The last straw was when my boss revoked a vacation day I'd been approved for well in advance -- for my dad's birthday -- because he was unhappy with my performance. I'd been trying to have a conversation with him but he was obviously not working in good faith. The project was simply too big for one person, crucial people had left, other people we depended on wouldn't pick up the phone, and I was obviously being set up as the fall guy. I also had some legitimate HR and legal concerns that the powers that be were also dragging their feet on handling. They wouldn't pick up the phone, and my emails only got boilerplate responses like "I've received your emails and will look into responding when able." I kept records; there were over 2 months of HR fobbing me off, in writing.
So the day I had my fathers birthday PTO revoked, I went home and checked my finances, and saw I had enough expenses to last me through the next year if I needed. I knew I wasn't looking at a years' unemployment, and I'd also had interviews lined up. So the next day, I went in, wrote a brief email to my boss saying something to like "I'm writing this email to inform you of my immediate resignation. I've left my work ID and keys with Jim Bob and my other work things can be found at my desk. I thank you for granting me this opportunity and wish you the best," and left.
That's a pretty minor example, but it's why I try to be frugal. If I had $4K monthly expenses and $10K net worth, rather than $1,600 in monthly expenses and $120K net worth, I couldn't have done that. I'd also saved copies of the relevant emails, so if this guy decides to pursue a vendetta against me I can make it difficult for him.
At the same time Malkynn is right in saying people can usually afford to speak up more than they do. At my current job, I say I'm not going to do stuff frequently, and my bosses love me. I never say "I'm not going to do that," or "it's not my problem," but rather "that's a good idea, but we're already committed to X, which should be much more profitable. If you think it's the better idea, I'll switch gears but I'm skeptical." It's a fine line and I wish that they offered classes on it in college!