Lets assume a world where everyone is a farmer. Everyone might have time to spare building their own homes, but roads wouldn't be built, and time spent on anything else could be spent on working more land to create enough food for more kids who might live long enough to farm more land. This kind of life is risky, a bad season could end multiple families all at once. Still, food is produced, value is added, and a necessity is satisfied.

Most other jobs don't satisfy a need as well as farming, so for other things we need to assume that needs are fulfilled and all that remains are wants.

Education isn't necessary but it is important in a society that builds on the efforts of the past. People have worked on and solved problems over time, and throwing that away would be a waste of resources, both what it cost to do the research, and a waste of future resources spent to re-solve the same old problems. The arts, storytelling and theater are important ways to convey ideas and morals, people who read more stories of other places, people, lives and experiences may be more empathetic and understanding than a person who has only ever lived in one place with only a few options when it comes to kinds of food, kinds of friends, and kinds of weather and terrain.

Then there's jobs that maintain, healthcare and roadworks, construction and demolition, that solve problems indirectly. Health might seem like a direct problem, but in reality healthcare is more about the society than the individual. If everyone had a fatal illness then healthcare would ensure society is as productive as possible given the circumstances. An outbreak isn't a problem for you, it's a societal problem with solutions that involve you, same way that food distribution isn't a thing farmers need to be concerned about yet must be resolved as efficiently as possible to ensure productivity.

And then there's parasitic jobs that feel like maintenance jobs, but aren't. Maintenance jobs resolve problems that are persistant, and disappear when new solutions are found. Parasitic jobs are eternal and rely on problems they contribute towards. Thatch roofing used to be a common maintenance job, but now is mostly gone as new methods were developed. Landlords and rentals are parasitic, because while reducing the cost of housing and increasing the amount of available housing would fix the problem, landlords can and do use their money to buy up available housing, it's a recursive effect.

Maintenance jobs can turn into parasitic jobs, the way that car production did. Initially cars replaced horses, but over time they made the option to travel via walking everywhere unviable through political pressure and money gained from sales of cars. Cities used to be structured where businesses and housing were combined in the same building, but now residential is fully separated for a majority of people, resulting in a need to own or use a vehicle to complete the last step of other maintenance jobs, like buying food.

The important part of what makes a job parasitic is that removing its existence would temporarily cause problems (since society was structured to involve them) but be quickly resolved once the structures supporting it are also removed. The reason residential and business is separated is due to regulations that can be changed, and once businesses and residential are mixed it'll be easier to walk wherever you need to go. As for landlords, removing them is harder because of housing prices held up by landlords, and so we'd have to wait for pricing to reset as the market falls apart. When it's no longer profitable to buy homes, they cease to be an investment vehicle, and simply become a place to live that decreases in value over time. Only reason housing gains value is the strength of the economy and scarcity of housing perpetuated by people buying houses in order to sell them again or rent them out. If corporations couldn't legally buy housing, how expensive would they be?

As for the question "what if there was only one job in the world", landlords would the one of the worst. If all needs were met and all people were landlords, then the need for landlords would cease to be since all people would have somewhere to live. Their existence requires some people be unable to live anywhere else for a smaller temporary cost, but still able to afford a long term higher cost if they end up with no other options. The other reason it's one of the worst is that the job isn't construction, it isn't repair, it's a person who sits between residents living there and the means to improve the quality of the building in the event of damage. Remove them, and the resident would be able to get repairs done directly for less cost. Only real barrier is the large upfront cost of buying a home outright, which leads to money lending which is another parasitic job.

There are probably other kinds of jobs than supply, maintenance, and parasitic, but they're good enough for constructing fictional societies with fictional jobs. A lot of stories are generic in a weird way when it comes to work, they don't differentiate what kind of office job a character has, for example, they kinda brush over the details fast and usually ignore anything beyond whether or not the boss and co-workers are nice. In anti-authoritarian stories the character might lose their cool and call out the business model, but again this doesn't take full advantage of the complexities these systems can have. A parasitic company might offer to deal with the red tape that a maintenance company can't afford to deal with, so long as they do something that benefits the parasitic company in the long run. A supply job has to choose to distribute to either a parasitic company that pays more, initially or a maintenance company that will not use pressure to lower costs over time.

The actual worst parasitic job is advertising. They often don't care if a product is legitimate or a scam, it's not their name that's affected once the scam is revealed, far as the audience knows the advertiser was tricked into it with boatloads of money. If ads didn't exist then word of mouth would quickly replace it. Advertisers generally don't produce anything of value outside of ads. Sometimes they do, and they use the money to fund those projects, but if there were only one job in the world and all other needs taken care of, what exactly would they be advertising? It's an existence that requires another business in order to exist, meaning its value is wholly dependent on someone else being more successful but simultaneously not good enough to get by without artificially spreading word about their own product, such as a scam. Maybe in the future advertisers will be seen as peddlers of scams and we can use morality and ethics to slowly get rid of it, but likely not.