What is the fundamental role of the Interstellar Medium (ISM) material found between the stars in the disk?

It is recycled material seeding the next generation of stars and planets.

The Interstellar Medium (ISM), comprising gas (mostly hydrogen and helium) and dust, is essential for maintaining the active life cycle characteristic of spiral galaxies. Stars form from this material when it gathers in cold molecular clouds. When these stars reach the end of their lives, whether through stellar winds or dramatic supernova explosions, they expel heavier elements (astronomical metals) back into the ISM. This enriched material then becomes the raw input for subsequent generations of stars and, crucially, the formation of planets, representing a continuous recycling process that distinguishes active spirals from older systems.