"To Helen" is more than a title, gang. It's a dedication! Everything in the poem (title included) is framed as a shout-out to a woman named Helen. But most critics and scholars are agreed that Poe was actually writing about (and to) a woman named Jane Stanard, not some random woman or classical reference. So why, then, does Poe call her Helen? Well, Poe was a big fan of ancient Greek mythology, which tells the story of a very beautiful Greek woman named Helen who was kidnapped by an Asian guy named Paris. All the Greeks banded together, sailed to Asia (western Turkey), and fought against Paris's people (the Trojans) for ten years.
The whole deal with Helen for Poe, however, isn't the kidnapping or the war or any of that stuff, but the fact that the name Helen is pretty much synonymous with beauty. In fact, it's been like that for a really, really long time. The mythological Helen is one of the most famous symbols of beauty in all of western literature.
Poe renames Jane Stanard "Helen" then, because, well, it's a lot more poetic than the name "Jane" (sorry Ms. Austen); it has a lot more symbolic weight, so to speak. Just the very mention of the name invokes a classic tale of a woman whose face "launched a thousand ships," stirring two nations to war for crimminey's sakes.
While it's hard to understand that sort of story from a modern perspective, put it this way: it would be like naming your poem "To Brad Pitt" or "To Megan Fox"—rather than "To Jennifer"—in order to emphasize the extraordinary hotness of your significant other. Sure, naming a poem after Brad or Meg sounds sort of weird, but the Greek Helen that Poe almost certainly has in mind would have probably represented the same things for him that Pitt and Fox do for us—namely, crazy amounts of beauty.