If your goals are appearance related, then I suppose when you achieve the look you want, you switch to maintaining.
Personally, I do Crossfit because my goals all revolve around improving my "general physical preparedness" (GPP): that is, improving my performance across all fitness domains, including strength, speed, endurance, agility, etc. I do it because it's healthy (which is inherently a life-long pursuit), it's practical (allows me to better handle anything life throws at me, from a new sport, to manual labor to a fight), and it's fun. In fact, the last point may be the most important: I see Crossfit as no different than any other sport and strive to get better at it. In the same way that you could spend a lifetime trying to get better at basketball or hockey, you likewise can keep getting better at Crossfit. Jordan and Gretzky were never "done" improving their abilities, and they were far better at their sports than I'll ever be at Crossfit.
Of course, on top of that, the "general" part of GPP gives me so much stuff to work on - every type of power lift, olympic lift, gymnastics movements, running, rowing, KB's, jump rope, climbing, flexibility, plyometrics, etc - that there is always something to work on. Every time I achieve one goal, I can always make it tougher or move onto something else. Find a weakness, make it a strength, rinse, repeat. I have a great life outside of the gym, but TBH, I don't see my time at the gym as a chore - I actually enjoy it and look forward to it.