Symbolism, Imagery, Allegory
It's a pretty striking image, right? Its significance as an image, in fact, is pretty close to that of the fiery furnace. We don't know where Daniel was during that debacle—sort of AWOL, we guess—but he gets his own personal version of the same thing in the lions' den. He's punished for almost the exact same reasons. Shadrach and co. wouldn't pray to a giant gold statue, and Daniel wouldn't pray to Darius.
The big take-away is that God will protect his worshippers even in the face of certain death. And he'll ensure the big reversal, where the conspirators (the trap-happy satraps) end up being the ones hoisted on their own petard (along with everyone else in their own families). Not only can God make fire not burn, but he can keep a bunch of lions' mouths shut if he wants to.