I believe that most (all?) of the greatest golfers in the world work regularly with teachers (aka Swing Coaches). I presume Tiger and Phil have put in at least a little bit of time studying the mechanics of the swing, and yet they seek professional help, and probably not just to validate their gifted play.
So... A lot of good golfers don't take lessons often or at all. Some, I'm sure, can indeed sort of analyze their "issues" and sort them out for themselves to their satisfaction. I guess "to their satisfaction" is the key there, if you're happy with your level of play more power to you!
A lot of pretty crappy golfers don't take lessons often or at all. 'Nuf said there...
I'm still a pretty crappy golfer, having just taken the game up again after more years than I like to contemplate too carefully. But I AM getting better. I've been working hard at it. And I've been taking lessons that have really helped. I have a long way to go, but I believe I will get to a level of play I am pleased with, and I don't mind being seen out on the range getting some instruction to get there.
So my answers to the poll: "Needed when you want to improve." AND "Needed when you feel you have 'lost' something." Obviously, if you don't feel you have 'lost' anything, or can find it again for yourself, and you don't really care to improve any more, you don't need to waste the money.