Hogwarts School of Witchcraft and Wizardry is "Unplottable" to keep its location secret, presumably to protect the school and its students from dark wizards, discovery by Muggles, or anything else that might threaten the school or the people within it. However, the nearby town of Hogsmeade is not similarly protected.
With the close proximity of Hogsmeade, (described as the only all-wizarding village in Britain) wouldn't it actually be quite easy to discover the location of Hogwarts?
Since a large percentage of English wizards, both good and bad, attended Hogwarts I think it is not accurate to say that the purpose of being unplottable is to prevent those who know of the school from getting there. Rather it is to keep it off the maps of inquisitive muggles, and likely to prevent magical scrying of the demesnes.
This is implied in the Goblet of Fire when Hermione lists it as a tactic used by the schools which try hiding from foreign Wizards.
“There’s traditionally been a lot of rivalry between all the magic schools. Durmstrang and Beauxbatons like to conceal their whereabouts so nobody can steal their secrets,” said Hermione matter-of-factly.
“Come off it,” said Ron, starting to laugh. “Durmstrang’s got to be about the same size as Hogwarts — how are you going to hide a great big castle?”
“But Hogwarts is hidden,” said Hermione, in surprise. “Everyone knows that . . . well, everyone who’s read Hogwarts, A History, anyway.”
“Just you, then,” said Ron. “So go on — how d’you hide a place like Hogwarts?”
“It’s bewitched,” said Hermione. “If a Muggle looks at it, all they see is a moldering old ruin with a sign over the entrance saying danger, do not enter, unsafe.”
“So Durmstrang’ll just look like a ruin to an outsider too?”
“Maybe,” said Hermione, shrugging, “or it might have Muggle- repelling charms on it, like the World Cup stadium. And to keep for- eign wizards from finding it, they’ll have made it Unplottable —”
“Come again?”
“Well, you can enchant a building so it’s impossible to plot on a map, can’t you?”
“Er . . . if you say so,” said Harry.
Harry Potter and the Goblet of Fire - Chapter Eleven
As can be seen, Hogwarts uses different techniques to hide itself.
Another proof of Hogwarts plottability is the existence of the Marauders Map. Hermione says in book 6 that the reason why the Room of Requirement doesn't appear on the map is because that room is unplottable. (Obviously implying that the rest of the castle is plottable.)
“I think it’ll be part of the magic of the room,” said Hermione. “If you need it to be Unplottable, it will be.”
Harry Potter and the Half-Blood Prince - Chapter Twenty-one
