See, this is precisely the reason why we see the fanfic near-polar opposite of the magical OC we don't care about: The no-magic story that uses canon HP characters we're supposed to already care about.

There's no such thing as magic, the story takes place in the normal world, in a normal school (or workplace); there's basically nothing of the Harry Potter world in such a story, but the characters all have names from the HP canon.

Basically, it's NOT a Harry Potter story and shouldn't be listed as such, but it happens...

It's done solely because the writer wants to write their own original story, but doesn't want to make the effort to build their own characters to the point that the reader cares about them on their own merits. Instead, they use the reader's preexisting emotional connection to these characters as a shortcut.

It also helps them jump straight into the story without doing the drudge work of so much as introducing the characters and showing you their situations. They just dive into the juicy part of the plot with a wave of the hand and, "He's Harry Potter; you know Harry already. Same guy, honest! Why drag things out with storytelling?"