Most prefabricated filters use a diffusion pattern that is uniformly distributed across the surface of the filter. This doesnt allow much variation and the central area is equally diffuse as the rest of the image. It works, but can be boring, as there is no relatively sharp area about the face.
By making your own, especially by smearing something that can be cleaned off, you can experiment with various patterns and strengths without permanently altering a filter.
If you dont like the effect, you can clean the filter and start over.
Some of the most useful effects are realized by smearing a thin film of petrolatum about the outer 40% to 80% of the filter and leaving the center clear. Then by choosing various aperture values you can tune the percentages of sharp vs unsharp image-forming light.
The clear center projects a sharp core while light passing through the smeared outer areas projects a diffuse, out of focus image onto the sharp core of the image.
Done right, this can deliver effects something like a poor mans Imagon. Close the aperture, and much of the smeared area is blocked from contributing to image formation so that the image is sharper and less diffuse.
Shoot wide open for the maximum softening. Intermediate aperture values give intermediate softening.