Article 370

article 370 was introduced to abide by the terms of J&K's accession. The accession of J&K was conditional.
India desperately wanted to prove 2-nation theory wrong and wanted a muslim majority region under its fold, also the strategic significance of Kashmir and willingness of Kashmiri leaders to have separate state for themselves (under protection of India) were the main reasons why India went ahead for accession of J&K.
When Instrument of Accession was signed, constitution of India was not ready. As per clause 7 of the Instrument of Accession, J&K was not committed to accept the future Constitution of India.
In 1949 november when all the princely state heads and provincial heads of Indian dominion were supposed to issue proclaimations making Constitution of India operative in their respective states and provinces, J&K refused such proclaimation refering to the clause 7 of Instrument of Accession. This is because the draft form of constitution refused separate constitutions for the states and J&K always wanted their own separate constitution.
There was a legal imbroglio in this situation, the accession issue was already with united nations and in such situations India has to abide by its promise. This lead to Article 306-A of the draft Constitution of India (which became article 370 in the actaul constitution).
Considering the overall situation prevalent, Article 370 was a major step forward at that time. The Article at least paved the way for the Republic of India to make several laws and provisions of the Constitution of India applicable to Jammu & Kashmir State beyond the strait jacket of the Instrument of Accession. Besides, Article 370 was conceived as a temporary arrangement, with hopes of a full integration in time to come.