Great question.
Finding MMM a few years ago helped me realize just how many OPTIONS I had available if I prioritized having FU money. Having a 'stache meant that I could decide I wanted to, say, move to Denver, and just up and do it, or push back a little bit at work when the boss man makes unreasonable requests (I've literally seen him bully people into canceling weekend plans to come in on a Saturday/Sunday to work on his projects, because of a crisis of his own making). So first and foremost, it's indeed about the freedom - within reason, I'd like to be able to spend my time and money as I see fit.
Secondly, it's about the position of strength. My parents have always led by example, but the message doesn't seem to have gotten through to the rest of their generation or most of mine with the exception of 2-3 of my cousins. I've never actually asked them about it, but I'm curious whether they see themselves as having some sort of responsibility for being the stewards of the family financial legacy. It could just be that neither of them had 2 nickels to rub together at times growing up, leading them to be ultra conservative, but I do think they take a fair amount of pride in the resource stability they have achieved and having the ability to help others right the ship if need be. I consider this line of thinking pretty graceful, and part of me wonders whether I should up my FIRE threshold a little more to build in the capacity to do the same.
Third, its about conservation and responsibility in the lifestyle. Becoming a stoic and consuming fewer resources and luxury goods has obvious financial benefits, but the unburned petroleum and reduction in plastic crap makes me feel like I'm making some small steps in the right direction. I subscribe to the adage that "defense wins championships" - financially, this means that reduction in spending enables quicker/more sustainable FIRE, environmentally it means not using resources at an unsustainable rate or breaking things that can't be fixed. I enjoy being out in nature, and I'd like to be sure that if I'm blessed with a future generation of mini-me's that they'll be able to do the same.
Fourth, fulfillment. There is an undeniable sense of achievement when one accomplishes something by "muscle over motor", whether that be chopping wood, biking to work, etc. FI may be a threshold to aim for, but as Nick Saban might say, it's about the process. Wealth is a habit, and the process of building it can be reduced to a series of habits and decisions. Since I started making big changes like moving out of the city and biking to work, selling all my old and underutilized stuff in a move toward minimalism, and eliminating wasteful habits like always going out for $10 drinks at local bars, the process has become a lot more fun and fulfilling. Sometimes when I have tough days at work, the bike commute will be the highlight of my day. It's easier to see the simple joy in life when all the distractions and superfluous bullshit gets cleared out of the way.
I'm still figuring out the details of what exactly I'd like my FIRE to look like, and what elements it will include. The goal is undeniably to buy back time, create self-actualization and personal improvement, with a side helping of charity. Would be nice to travel and see more of this blue marble, too.