In D&D the term campaign most often means a setting with an ultimate goal. It is a term taken from the military, as in military campaign, since miniature wargaming was the structure from which RPGs emerged.
For many, campaign is a unique world/realm, and if it flops, the campaign is scrapped, and the would-be DM, must start over from scratch
For others, campaign can mean the setting a given DM uses no matter how many groups of players flop or thrive. Talented DMs may run multiple gaming groups in the same general area in their game world on the same time line. Others may separate their players and each is in a unique instance of the game world, like parallel universes. For the first, the players can easily switch which group they are with, for the second, some sort of magical portal transports them to the other timeline.
The type of campaign that encompasses a world/game setting is also called a milieu, which means setting, and can have multiple campaigns run simultaneously or in sequence, or both.
The idea of a campaign as a one-shot world is from the heavily story driven type of play where there is an ultimate goal to the setting and once it is achieved, there is no sense to go on. To continue play requires a new setting for a new campaign. I can see where some might like this type of play, but that is an enormous amount of work.
It makes more sense to me to have a game world that can offer multiple locations for players to adventure. In this way, the DM can develop one setting, and spend the rest of their time fleshing out the details of tombs, caverns, lairs, treasure, magic, dungeons, villains, and monsters in a given area.
This can work for a fantasy world, where a continent or sub-continent sized area is roughly sketched, and the details of the starting area are worked out and the players are let loose to figure things out. For a space based science fiction game, why plan out multiple start systems and their planets, only to have to do it again? For a post-apocalyptic setting, why plan out things only to never use them again?
Obviously if the DM has an idea that he thinks is cool, but flops, it can be scrapped if it is beyond salvaging, or it can be re-worked to be cool. Keep the good ideas and toss the bad ones.
In my game, I have things going on the players only have glimpses of, but have yet to investigate. So far, I have not gone beyond ideas and scribbles of notes. The game needs to mature and simmer some more to determine if my ideas for the machinations of yet to be discovered bad guys fits with what develops in play. I won’t put too much time into some of it until the players are about to discover it. This makes for a fluid and more organic game setting that is more likely to be fun for all.
