It's just convenient not to have to wash the cup.
At work I have a glass mug for coffee, but I still use paper cups for water. If I replace it with glass it means I will now have 2 things to wash.
At work I have a glass mug for coffee, but I still use paper cups for water. If I replace it with glass it means I will now have 2 things to wash.
What about the dish washer? Isn’t that convenient? If it is environmentally friendly or not that’s another question.