A brush that blurs brush strokes. Its called “smooth” in blender and Zbrush.
Its an essential tool when handpainting textures or masks, especially for more stylized textures.
You could use a blur filter in some cases I guess, but it might be easier for quick fixing of paint strokes and more convenient for those less technical artists, that just do it all by hand.
Ideally this would not be a separate brush, but rather a brush modifier you could access for example by pressing “shift” or another key, or you´d have to switch it like you can switch between "paint " and “erase” with B and E