If the selection in NIT's or IIT's on the basis of JEE main or JEE adv. Mark only then what is the purpose of minimum 75% marks obtaining .

I think it will change soon

If this criteria will not be there than most of students will not focus on the writing style and the other subjects which are there in the board like English and physical .
Some students qualify the mains on basis of formulas and also in jee there is multiple choice questions.
You can not check the ability of any student from  only on this paper.
Boards are to check your conceptual and writing skills.
And there are lot more. To judge.by boards
Good luck.

It is given so that u give some priority to board exams also else students will only prepare for competitive exams and neglect board exams preparation

Counselling for NIT's is done on Basis of JEE Mains.
And For IIT , it is done on The basis of JEE ADVANCE.

Minimum 75% is required in Boards for the Admission.