The reasons for having your own pewter or metal tankard are manifold.
1) It stops other people drinking your beer by "mistake". 2) If it has a lid it stops people dropping fag ends or wasps in. 3) A metal tankard can be carried around without the risk of it breaking and stabbing you in the vitals. 4) If you have a proper British Standard tankard you know you are getting a pint, especially if you don't drink frothy beer. 5) You have the option of slowly poisoning those you don't like 6) You can get a 4 pint tankard, ever seen a 4 pint glass beer mug?