Before you can stain, paint, or coat a product properly, the surface needs to be right.
Brushing helps with that. It removes fibres from machining, opens the wood grain, and can create texture if needed.
But not every product reacts the same. A flat panel is different from a profiled part. Oak behaves differently than MDF.
That is why the brushing process needs to match your product. When it does, you get a consistent surface and your coating process becomes much more predictable.