filter brushes is very useful in applying filters to the drawings as we wish and not apply the whole filter to the image, dynamically the filter can be applied.
Hello @sundiamond and welcome to the forum 
There are already filter brush presets such as the Soft Blur and Filter Blur brush presets that you can find in the Brush Presets docker.
They use the Filter Engine that you can find in the Brush Editor and can use it to make a brush preset that can apply many different types of filter:
Is that what you mean?
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Filter masks are another way to apply filters to only parts of an image.
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