There's nothing wrong with others paying (or bartering with) you for a skill you have. You do have to be comfortable with that arrangement. Often I'll do something for free because we're not talking large sums of money lost and that way I maintain control over my availability/time commitment.
I think the definition of retirement has changed over the past couple of decades. Back when most people lived to only to their 60s or maybe 70s, retirement typically meant being old enough to do little more than rest or do non-strenuous activities. Now people are retiring in their 50s and most can expect another 20-30 years of activity before having to slow down. That's a lot of time to kill and really can put some pressure on retirement savings. So doing something different or concentrating on a subset of your previous career makes sense and, apparently, seems like a good idea to many people.
DW plans to retire in the next two years and wants to start her own business doing a subset of what she does now (the part of her job she finds the most interesting and rewarding). Her goal is to occupy some hours (the hours not spent with me or family or hobbies) and make enough money for traveling and luxuries, so it won't be a 40-hour-a-week slog. It may not even be a 20-hour-a-week deal. I'll be the back office (billing, IT, etc.). We'll learn more as she gets into it. We'll be barely 60; plenty of time left to enjoy life.