Keyboard key W only tells whether your character is moving or not moving. So lets say your movement code looks like
if(isKeyDown(VK_W))
character.position.x += 10;
This will move your character 10 units every frame. With 60 FPS that's 600 / second, with 1000 FPS that's 10000 / second. This is why you multiply by delta.
Mouse events, on other hand, tell how much it moved, not only whether it's moving or not. When player moves mouse 1 inch you get event it moved 10 units and it doesn't matter whether you rendered 1 or 100 frames during that time.