Like running your hand over a piece of wood that appears smooth until you touch it.
The rifling inside your barrel is like that piece of wood, not only could it be smoother, but it also will collect more fouling, if left alone.
The top two (proven) least expensive techiques which improve accurarcy the most are barrel lapping and trigger jobs.
Studies with the same rifles (using the same everything) with groups measured before and after each application have proven results.