Glasses can be properly cleaned using your boiling water unit. Pour boiling water into your glass to sterilize it. Let the boiling water overflow the edges to ensure the sides are cleaned also. Try doing this with your plastic bottle of Mt Franklin and you’ll end up with a melted mess. You can’t properly sterilise single use bottles, so they can quickly become a breeding pit of bacteria.
The chemicals used to manufacture plastic bottles and cups (less so with expensive re-useable ones) can leach out into your water. By comparison you can happily leave a sealed glass bottle of water in the fridge for months.
Compared to plastic cups, you can add in lemon or lime pieces into a glass without the risk of it becoming discoloured or degraded.
You can at least pretend that you care about the environment. Glasses can be safely washed and re-used hundreds of times, whereas single use plastics are causing horrendous permanent damage to our environment. Plastic bottles in landfill can take 450 years to decompose.