This is a very relative question. See if we see the point then if you want to study in IIT's then you have to qualify JEE MAINS for JEE ADVANCED and if you want to study in NIT's/IIST's then its become very crucial. But suddenly this doesn't mean that JEE MAINS compulsory if you want to get into this institutes then you have qualify it. Now there are many private colleges which also offers admission on basis of JEE MAINS. This are some of the advantages. But if you have no such thoughts then nope! JEE MAINS isn't important to pass. You can qualify an state level entrance and you can study in state government engineering colleges.

Thus we arrive to this point that why NIT/IIT? See for admission in NIT's/IIT's such kind of exams are taken with such difficulties and competition because NIT's and IIT's are the best for engineering in India. In terms of infrastructure, placements, research, faculties these are the best. Again this doesn't mean that state government colleges are bad, what I am trying to say is that in NIT's/IIT's you will get greater exposure to your field. so its better to atleast attempt this examination. Also if you prepare well for JEE MAINS then cracking any state level engineering exam will be relatively easier for you. So while you are preparing try to prepare for the one which gives you more exposure.

To know more about JEE MAINS visit the link below:

https://engineering.careers360.com/exams/jee-main

For state engineering exams visit the link below:

https://engineering.careers360.com/articles/top-engineering-entrance-exams-in-india

I hope this answer helps. All the very best for your future endeavors!