Well, to start with, the title, "Dover Beach," tells us where this poem takes place. Or maybe it tells us where this poem takes off from, since it eventually launches into other worlds of time, space, and imagination.
But that starting place matters. The sight of the beach at Dover and the sound of the pounding waves inspire the speaker's wide-ranging thoughts. Putting the name of the place at the top of the poem is a way to underline its importance.
It's probably also worth mentioning that the cliffs of Dover are hugely symbolic for the English. They represent the English nation, and they were a familiar sight for travelers coming and going to Europe. We're willing to bet most English readers of this poem would get a mental image of Dover, a kind of anchor in the world of the poem, just by reading the title. That gives Arnold a boost when he's laying in those crucial visual images at the start of the poem—it makes it even easier for us to put ourselves where the speaker is.