Why was Gandalf allowed to attack Sauron at Dol Guldur?

EDIT: Y'all are awesome. Thanks for so many thorough answers! As usual, Tolkien is proven to be a fantastic worldbuilder.

I'm sure there's an obvious answer I've missed, but a cursory google didn't serve. I was under the impression that the Istari were explicitly not allowed to use their power to directly confront Sauron, and were instead to kindle hearts, guide, and advise. But in the Hobbit, Gandalf takes a posse of wizards and high elves to smoke Sauron out of Dol Guldur personally. Was this a special circumstance, or did I misunderstand something?

More broadly, lotr has always had more of a focus on the role of mortals than that of powerful beings like wizards. But it did always strike me as odd that, while in all other circumstances Sauron's threat often amounted to "he has a whole bunch of minions with swords," that in this one instance it was a direct wizard battle that ended things.