LOST was well known as a strange and sometimes completely nonsensical show. It was absolutely full of odd appearances, apparitions, dream sequences, and flashbacks. Occasionally, these were important pieces of information that fans could use to better understand the intricate, confusing story playing out each week.
Sometimes they were polar bears.
The polar bears appeared for the first time right at the beginning of the series, when they attacked the castaways in the jungle. Initially, they were meant to indicate just how strange this island really was, something appropriately random to prove they were not in Kansas anymore. Instead, they became a running motif throughout the show (or was it a gag?) more than likely intended specifically to throw fans off, because I'm convinced LOST was actually a government experiment to see how much narrative torture our brains could endure. They were significantly insignificant.
In fact, the LOST polar bears are probably the best metaphor for how fans should treat a lot of those crazy bits of nonsense that often gave them headaches from all the debate over their obvious importance. Were they an indication of Walt's burgeoning superpowers? Why did the DHARMA Initiative bring them to the island in the first place? Do you think it was difficult to carry multiple polar bears in a submarine? Where was THAT episode instead of that weird "how Jack got his tattoos" one? Or that one where Allison Janney basically gave birth to God and the Devil? Don't worry, it's just a polar bear.
Hell, use the polar bears as a metaphor for your own life. Encounter an obnoxious person out in the wild? Don't worry, they're just a polar bear. A strange turn of events set you on a new path in your career? Polar bear. Having a terrifically bad day you couldn't possibly bounce back from? At least you're not on a deserted island being attacked by a polar bear.