What is the critical function performed by the density wave as it passes through the interstellar medium?

It compresses the diffuse interstellar medium, forcing molecular hydrogen gas and dust to collapse gravitationally.

The primary importance of the density wave mechanism lies in its ability to act as a trigger for new stellar birth. As this wave, characterized by higher gravitational density, moves across the galactic plane, it interacts with the vast, diffuse clouds of interstellar material, primarily molecular hydrogen gas and dust. This interaction forces these clouds to slow down and undergo significant compression. The resulting increase in density and pressure within the cloud eventually overcomes the internal gas pressure. When gravity wins this battle, the cloud collapses inward, initiating the process that leads to the formation of new stars and star clusters. This compression step is the essential physical event required to create the brilliant blue light source that defines the visual spiral arm.