It seems to me we’re circling round the truism that “players don’t like to have PCs who don’t fit the game” - even if you can swap out a PC who doesn’t fit for one who works better, character generation is effort, and in an XP-based game a replacement PC may fall behind the others in spotlight-grabbing power. That’s all fair enough.
But that’s not the only reason I like a quick campaign description as summarised in the thread title. Some of my players are quite picky about the sort of thing they want to play, and if I come up with several campaigns at this level of complexity I can get the unacceptable ones rejected before I go into more effort building them. Also, as I believe Bill suggested in his campaign prospectus article back in Pyramid volume 2, once players have said in some form “yes, I want to play in this campaign rather than that one” there’s slightly more commitment than if it’s just “I will go along with whatever Roger runs next”.
An example of a setting that doesn’t do much of this is GURPS: Tales of the Solar Patrol from 2008. (Which I actually rather like the look of; I’m using it as a bad example here because it gets most of the other stuff right.) 45 pages of world background, technology, character templates; three of campaigns. The Solar Patrol campaign, the default mode, gets less than half of one of those three pages. Now, yes, some of the locations suggest things you could do, but there’s an awful lot of work needed to go from reading this book to blowing away pirates with your atomic pistol.
So if I were proposing to run a campaign of this, I couldn’t just say “take a look at the book”; I’d have to say something like “you’ll start out as cadets, then graduate to the Patrol proper; you’ll be the crew of a patrol ship, sent up against a variety of threats and expected to handle them on your own.”
This then veers into another campaign-building truism: if when I’m designing it I can’t readily think of about ten things for the PCs to do (“adventures”, if you’re using that model), it probably doesn’t have the legs to be enjoyable. What are the things the Space Patrol does? Fight pirates; do colonial things on Venus; maybe go covertly to Mars; fight agents of the Overlord of Jupiter; and…? This is the point at which I stop thinking I want to run a Solar Patrol campaign, because I feel it’s going to wear thin quite soon.