Siachen is the only piece that keeps Chinese and Pakistanis to openly attack India at it's north. And also it stops the enemies from attacking Kashmir and Ladakh.
When the 1947-48 war happened between India and Pakistan, the Cease Fire Line was delineated under the Karachi Agreement of 1949 only up to a point on the map known as Point NJ 9842. The area to the north, being highly inaccessible and glaciated, was not delineated, but the direction of the CFL beyond NJ 9842 was unambiguously stated as ‘thence north to the glaciers.’ The same happened when the CFL was replaced by the LoC after the 1971 war.
In 1984, there was a confirmed report by intelligence of Pakistani army attacking Siachen. The army without any delay started Operation Meghdoot and before the Pakistani army reached Siachen, Indian troops covered all the posts. Indian army occupied the Saltoro Ridges which runs parallel to the Siachen glaciers on its western side. That have been the actual line of control since then.
Pakistan cannot actually attack it and win, hence they choose a much easier option of talking on the table and negotiating.
