Opening lines

What's the point of an opening line? Is it to intrigue someone so much that they must continue on? Is it to set the tone for the rest of the book?  Is to make someone stop and think or pay attention right from the start? Or is to paint a picture in which the story will unfold?

Perhaps it's a little of each of those things.I've been practicing writing opening lines for images posted on Twitter by iAuthor.  

Here's an opening line I think will work in a future novel I am planning to write.

"My past held a very bright future. Then, life cast its shadows on my prospects." 

My story is going to take place on land but told from the perspective of being at sea.  It's going to reach into the deepest darkest places of someone's soul, but it will come out into the light in the end. At least that's the plan.