Hi,

The admit card has whole information about the examination you have registered. The information like your name, roll number, name of examination, center of examination, passport, time table is mentioned on that. 

It is important because it is used at the time of examination and as the required information to fill in your answer sheet. It is like ID proof that you have registered for that particuler examination. 

So, it is important without which you are not allowed give the exam.

Hope it's clear. All the best!!