Glass teapots show the tea brewing, so if you are a "by color" tea brewer you can tell exactly the color to determine when you want to pour.
2. Glass pots often have a little indent in the pour area which prevents spilling when pouring.
3. Older ceramics and antiques can have lead, cadmium, and even radiated glazes which are extremely toxic. Glass does not.
4. Glass teapots can be quite beautiful in the brewing process are used to display blooming flowers or herbs during the process.