A Gravitational Redshift is a Redshift due to the source being near a massive object (rather than due to a relative Radial Velocity between observer and the source). Instead of slowing down, as would a massive object following that path, Electromagnetic Radiation redshifts. Higher mass and density increase the effect and Black Holes, Neutron Stars, and Galaxy Clusters can produce a significant redshift. Stars can produce a significant redshift in their own Spectral Lines.
A measured redshift can be useful observation data, e.g., to help determine the mass of an object, or to detect and analyze the implications of the Sachs-Wolfe Effect.
Sachs-Wolfe Effect (SWE)