Why did Gandalf only take out his magic powers against the Balrog in The Lord of the Rings: The Fellowship of the Ring? Why not before? Could he have used that kind of firepower earlier to the Fellowship's benefit?
One of Tolkien's themes is the appropriate use of power, and in his universe, power comes from your inner nature. You cannot—or should not—obtain more power through any artificial system, because that just distorts your nature.
For example, we have Aragorn who usually prevails just by showing up and being all Aragorny. He doesn't make speeches, he just exudes this natural authority (or even semi-supernatural powers) that make people want to follow him. This is nobility, in Tolkien's view—some people are just born better. Others, like lesser men and Hobbits, have a lower status, but can play their roles with equal faithfulness to this principle.
So, what about Gandalf's power? If you read the supporting materials, you learn that he is really an angelic being clothed as a mortal. The Balrog is a being of the same order. So, in Tolkien's morality, Gandalf can reveal all of his power when in battle with an equal—the Balrog, or Saruman. In fact, when Gandalf gets his promotion to being the White, he has the natural authority to even cancel Saruman's power.
It would be immoral for Gandalf to use his power directly against lesser beings, even when that would do good. Instead, he must use his power more indirectly, to convince the people of Middle-Earth to unite.
Saruman is depicted as having succumbed to this temptation. Frustrated with the disunity among the peoples of Middle-Earth, he decided to create an artificial system that would be strong enough to defeat Sauron, namely his industrial revolution around Orthanc, yielding the abomination of the Uruk-Hai. And as he became interested in power for its own sake, he became a tool of Sauron.
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Gandalf is both meddeling and hands-off, this is part of what makes him an interesting character.
For instance, for as powerful as he is, don't you think he could have devised a better way to get to Mt. Doom rather than just walking/horse back? Sure, but the journey wasn't up for him to decide, it was for the rest of Middle Earth.
The Balrog, if you look into Middle Earth's back history, is actually more related to Gandalf than the rest of the beings on Middle Earth, both having been angelic beings at one point. Since this balrog was going to interrupt the happenings of Middle Earth Gandalf had to seriously intervene.
Gandalf has always been powerful, but he only expresses that power when absolutely necessary.
This question has a very large answer.
Actually, the Valar (powerful angelic beings living in Valinor across the Great Sea) had decided after the Akallabeth (fall of Numenor) to not interfere in Middle Earth directly. However, due to the rising power of Sauron, they sent six to Middle Earth to assist the Free Peoples - first Glorfindel the Elf, and then five Istar ("Wizards") who were actually Maiar (lesser angels). They took the form of old men as they landed. They were Gandalf, Saruman, Radagast, and two Blue Wizards. They were forbidden from using there true powers by the Valar, as they were NOT to take direct action against Sauron.
Now, the Balrog was not in Sauron's service. It was a fallen Maia in the service of Morgoth (the earlier Dark Lord). Morgorth's fall had seen most of these devils destroyed, but some survived, and the Balrog, better known as Durin's Bane, hid under Moria. After being aroused by the Dwarves, it drove them out, and gravitated evil toward itself (including orcs).
Since the Balrog was not allied with Sauron, Gandalf had the permission to use his full strength to destroy it, especially as they had equal powers. That is why he only uses his "magic" powers against Durin's Bane.
It was due to this restriction on his innate power that Gandalf could not use his powers before, ot to the Fellowship's benefit.
The Balrog was a powerful magical creature and was a "foe beyond any of [them]". In this situation Gandalf had to turn his dials up as it was the only option.
His mission was to inspire and assist, he was not permitted to take over and fight everyone's battles for them.
Interesting that you should say that. In the books he actually does. In the snow when they were passing over Caradhras, the rest of the Fellowship, tried and failed to make a fire, so Gandalf reluctantly uses his magic to start a roaring fire by which the fellowship could warm themselves. Gandalf is uneasy for the next few hours because he believes that it will make it easier for the Orcs to track them.
His exact words were:
"I have written 'Gandalf is here' for all to see."
So my understanding, metaphors aside, is that he uses his magic sparingly in fear of being tracked, there are after all, only four good wizards in the world at that time.