Why do I need "as V?" ?

gistfile1.txt

import Cocoa

private struct Bin<V> {
	var key: Int
	var value: V?
}

public class Thing<V> {
	
	private var bins = [Bin<V>]()
	
	public func value<V>(key: Int) -> V? {   // Answer: remove the <V> because it is not needed and shadows the outer <V>
		precondition(key != 0)
		for index in 0..<bins.count {
			let bin = bins[index]
			let value = bin.value
			if value == nil { continue }
			if bin.key == key {
				return value as! V? // Compiles OK
//				return value		// Cannot convert return expression type error
			}
		}
		return nil
	}
}