will getting qualified for jee advance confirms a seat in nit

It all depends on the cut off list of the NIT, actually every year it keeps changing and the standards too go high every time. So, I suggest you to check the cut off list of the particular NIT where you wish to get admission, and for sure yes just qualifying does not serve any purpose, one must be among top most candidates. There can be other options as well where you can try your luck to get an opportunity of admission, they are-

Maharaja SayajiRao uni, Vadodra

Lovely Professional Uni, Phagwara, Punjab

College of Engg. Pune

LNM Inst. of Tech. Jaipur