The Smart Blur effect blurs an image while preserving lines and edges within the image. For example, you can use the Smart Blur effect to smoothly blur shaded areas while retaining crisp outlines of text and vector graphics.
This has a unique effect, and can help remove lines and can also create great black and white masks for you.
Radius: How far the filter searches for dissimilar pixels to blur.
Threshold: How different the values of pixels should be before they are eliminated.
Mode: What parts of the image receive the blur. Normal specifies that the blur be applied to the entire selection, whereas Edge Only and Overlay Edge specify that the blur be applied only to the edges of color transitions. Where significant contrast occurs, Edge Only applies black-and-white edges, and Overlay Edge applies white.
From the Adobe Help Site(https://helpx.adobe.com/after-effects/using/blur-sharpen-effects.html#smart_blur_effect)
I have found that unless the radius has any size about 2, it will not blur regardless of the value of the threshold. So all previews have a threshold and radius because that’s just how it works.