Even if you love your current job, or you don't think early retirement is for you, because you love the idea of working in general, there are arguments to be made for saving and investing as if you were to retire early.
1) you may change your mind
Even if you're early in your career, and you love your job, you may not feel the same way 15, or even 30 years from now. Saving as if you were going to retire (becoming financially independent) gives you options. Once you spend the money, your options will always be diminished. Unless you're confident of your life's paths and you're spending towards it, saving and investing your money defers having to make a decision for your future now
2) Your future earnings may not be entirely in your control
Steady, reliable, long-term employment seems to be becoming harder to come by today. Work seems to be becoming increasingly automated, and people, if needed at all, become part-time cogs in the machine. Even if you have reliable employment with a pension, what Is to say that you want to tie yourself to that company, or that it will be around long enough to pay your pension? You may desire to adjust your lifestyle to be more flexible, by becoming a freelancer or running a business, but your income may thus not be as reliable. Thus, if your require most of your income to live your lifestyle and save little, you will be stuck to your job.
3) Compounded growth in the early years can set up you for life
Most people save for retirement backwards. They contribute little early in their careers, and more as they get older. But from pure numbers perspective it doesn't make sense. Contributions made earlier have much more time to compound, requiring only minimum contributions later on if you desire to go the traditional retirement route. If you even lived saving 50% percent of your income for the first 5 years, and didn't touch it or add to it until a normal retirement age, you could easily have an additional million to retire on.
4) it gives your life focus and discipline
Spending less forces you to make decisions about your priorities. Your decisions will be less about which toy you wish to buy next, but more about the direction you wish your life to take. You will be forced to discover happiness outside of consumption, and to really distinguish needs from wants.
5) it gives you the freedom to pursue your dreams and to shape your own destiny
If you don't need money, you have the freedom to create whatever pursuits you like. You can do whatever you want, even if it does not pay much or anything at all. You can volunteer with a pursuit you actually consider meaningful. You can travel where you want, when you want. You can study what you want, when you want, if you want. You can learn a new language by living in the country it's spoken.
Ultimately, you will not be able to shape your own destiny if you depend on money from a job to shape what you do.