Truth is, played correctly, you can basically get a free extra draw every turn. It's also not a Legendary Land, meaning you could play more than one at a time. This is why it's limited to a single copy in Vintage.
It's a bit skill-intensive to play optimally, but Legacy and Vintage are already very skill-intensive formats. It's just too good for a land that comes into play untapped and can also provide you mana if you can't get the extra card draw. It's just a really great card. Best part is, it's not even a rare. Even though it's an uncommon, though, it's on Magic's Reserve List of cards that can't be reprinted. It's always going to be one of the more expensive Magic cards out there.
Have you ever played Library of Alexandria?
~ Phoenix <3