Question

Why is the mirror of the dressing table always a plane mirror

Answer 1

so that when we dress ourself in the mirror, we should b seen the same what we r.
some mirror show short, tall, fat, thin, so this is not shown by plane mirror.
and also to get us accurate image as same size as v see

Answer 2

A plane mirror gives an actual image.meaning same size as of that of the object.
Also the image is upright.