Soft filters are often used in portraits or flower photography, as they can blur light and "soften" the atmosphere.
The same effect can do great in starry sky photography. Soft filters' peculiarity is that their "softening" effect will be stronger as stronger is the light passing through, and they are thus more effective when shooting under back light or oblique light.
There are various types of soft filters, ranging from low effect to high effect, or peripheral effect, ready to suit any of the photographers imagined ideas.
Also, follow the article on how to use soft filters to capture city illuminations.