The purpose is learning to identify which openings work, and which don't. The opening sentences can make or break it for a reader. It could be the one thing that makes a reader put down your book. So you've got to get it "just right."
There are so many choices to consider -- do you open with dialogue? Description? Action? Narration? If you're unsure, try each of all of the above, and see which works the best. Don't ever be afraid to experiment with an opening scene.
For next week's assignment, I've picked one of my favorite openings to read aloud. Here it is, by the brilliant Elizabeth Berg (Open House):
You know before you know, of course. You're bending over the dryer, you're pulling out the still warm sheets, and the knowledge walks up your backbone. You stare at the man you love, and you are staring at nothing. He is gone before he is gone.