When you have a large window with a staircase in front, it can be a detractor from the amount of light that you are getting into the room, and negates the point of having such a large window.
They used American White Ash to brighten the room to make it look as spacious as possible. In combination with the open treads, the client wanted glass balustrade. This has similar benefits to the staircase being open plan, and if this had been spindles instead it would of closed the hallway off and made it feel smaller. The glass balustrade is fixed to the handrail and baserail.