In Swift, why have "as" keyword. What does it benefit? Why did they put in?

I'm aware of what "as" keyword does and how to use them. But I'm more curious in the architectural side of the "as" keyword. Why is it there? What's the huge reason? What does it help?

Example:

var objectData:NSData = NSUserDefaults.standardUserDefaults().objectForKey("myKey") as NSData

wouldn't it be better if it's:

var objectData:NSData = NSUserDefaults.standardUserDefaults().objectForKey("myKey")