I mean in jee main and advance

Then it is compulsory. Get it done. For jee main you can't register if you don't have one. So i suggest you go to the nearest aadhaar centre, get the aadhaar application done, they will give a receipt with aadhaar enrolment number, use that to apply for jee main. In the meantime aadhaar will be done within the month so you won't have any trouble for admission later.