What if I won't write jee advanced paper,even though I cleared mains paper

According to your question you probably mean to ask that you have qualified for jee advanced and what if you don't write jee advanced? So I would like to tell you that you will not get an opportunity to apply/take admission in institutions which are asking for jee advanced scores like iit's this is the the only problem you will face other wise you will be able to get admission in all institutions asking for jee main scores like nit/iiit/gfti you may participate in josaa counselling for the same.

Hi Aspirant ,

No , it is not mandatory to give jee advanced . If you got good IITs or NITs so, then its only depend on you. However since you have cleared the cutoff and are elligible, there is no harm in giving the exam. Infact, you still have a month to go for advanced and studying with a proper strategy will definitely lead to a huge improvement.
And of course, most other engineering entrances are easier compared to the Advanced exam and this preparation will help you to do well in all the other exams as well .
JEE Advanced exam is conducted after the JEE Main result.

Hope this will help!!

Good Wishes for your bright future..