Making tea involves three types of vessel. The first is the kettle, in which you boil water on a stove. The second is the teapot. This should be somewhat heated with warm water (which is then thrown away) before you put the tea leaves in and transfer the boiled water. This is so that the boiling water doesn't instantly cool down when it contacts the cold pot. It may also help prevent disasters like the boiling water causing the pot to break thus wasting tea.
The third type of vessel is, of course, a half pint mug.
It should be noted that ISO 3103 disagrees. This is because it aims to define a precisely repeatable process. Swirling a small amount of warm water around in the pot isn't repeatable.
update: see also The Russian Tea HOWTO for a different tea-making tradition which nevertheless includes warming the pot. Daniel is a very clever hacker with a splendid sense of humour so is obviously right.