Article 370 is in Part XXI of the Indian constitution "Temporary, Transitional and Special Provisions". It was intended as a temporary measure.
It was introduced in the 1950s when the Princely States were considering joining India. These states, which had been fairly independent under the British Raj, were given the options of retaining their partial independence, or acceding fully to to the Indian Constitution.
The other princely states joined India and became provinices under the Indian Constitution. J&K chose to give the Constituent Assembly the right to decide whether to join as a regular province, or form some other kind of arrangement with the rest of India. Article 370 removes the full requirements of the Indian Constitution from J&K, until such time as the Constituent assembly can decide on what it wants to do.
It was intended as a temporary measure, in the expectation that the Assembly would probably vote to join India (or adopt some other constitution), at which point the President of India would have the power to declare that the article would cease to apply.
However, war with Pakistan changed things a bit, and in 1957 the Constituent assembly dissolved itself without either approving the Indian Constitution nor writing its own constitution. So we are left with the situation of an article which states:
the President may, by public
notification, declare that this article shall cease to be
operative or shall be operative only with such exceptions
and modifications and from such date as he may specify:
Provided that the recommendation of the Constituent
Assembly of the State referred to in clause (2) shall be
necessary before the President issues such a notification.
But there is no "Constituent Assembly of the State" that is competent to make such a recommendation. Thus the constitutional mechanism that could allow for this temporary article to be laid aside does not exist.
The article was intended to be temporary, but without any mechanism to cease its operation, it has become part of the permanent Indian Constitution.