I'll admit exceptions to the backstory claim: I enjoyed the appendices to Lord of the Rings just fine. But Tolkien was a good enough story-teller to make decent stories out most of them, and still leave plenty of mystery for flavor. 99.999% of the rest of us aren't and don't--the result is at best mediocre, and typically just bad.
There is no discipline in backstories. Got an idea? Chuck it in. No need to worry about pacing. And if there's a minor inconsistency in the real story, don't worry about it; a little creativity here and there in the backstory will make the inconsistency go away and leave the story utterly logical with all puzzles solved.
Suppose the secretary at the agency is a blonde when the PI arrives and a redhead when he leaves. This glitch is no mystery to the backstory writer: it is because she made a quick dash to the hairdresser in the meantime, which she could do because the company pioneered ultra-flex time, which the young heir pushed for, because he could not abide the regimentation of his childhood, which had been a family tradition since the Norman Conquest, which ... has nothing to do with the missing wife the PI was there to learn about. In the real story.
Niven's Ringworld series suffered badly from this. They aren't really backstories, but that's what drives them. Niven typically writes science fiction mysteries, where there's a puzzle that needs to be solved. Fine. But the puzzles get pretty cramped when you have to stack them inside fixes for mistakes in earlier stories, and feel you have to tie loose ends together everywhere. It turned out that a bare ringworld is unstable. That has implications for the design, and the tragedy of the commons means that that design feature will be misappropriated--all quite logical, but it feels forced.
Remember Dune and its (often iffy) sequels? OK, now do you remember the dreadful prequels? Prequels typically read as though the author is coloring in the lines rather than adventuring. Granted, there are sometimes irresistible possibilities: a minor character fit for a different set of adventures, but in those that come to mind the story was the better the farther away it was from the original. Zelazny didn't do too badly with Dilvish, for example. But Haggard's Ancient Allen (and apparently a lot of his similar works) was cookie-cutter.
Is it fair to say that a backstory is to a story like a blog is to a book? A story usually takes a lot more work...