A superhero's only as good as his villains, and an interesting villain is even more important when you have books with a single villain fighting an entire team of heroes. Not only does the team's enemy need to be powerful enough and clever enough to pose a legitimate challenge, they also have to be as, if not more, complex as the heroes. And that's true most of all for Magneto.
From the Jewish ghettos in pre-WW2 Germany to ruling his own nation, he led an interesting life, to say the least. Magneto has been considered everything from a terrorist to a savior, and acted the part accordingly. Unlike many early comic book villains, Magneto is far from the usual petty criminal or would-be conqueror with a silly costume and a gimmick. Magneto may be the X-Men's longest running antagonist, and also one of the oldest anti-heroes of the Marvel Universe, but there's still plenty that we don't know about him.
Magneto has never been easy to categorize, which is part of his charm. And as the oldest, and one of the most popular, recurring X-Men villain, even in his absence he manages to make his presence felt across the board. And after half a century of appearances in print and on screen, there's an awful lot of things people have forgotten over the years.