The downside of working with a publisher is that they typically receive half of all royalties generated. This is what's called a publisher's royalty, and it's how publishers make their living. Publishers typically also split licensing fees with writers, which is a one time fee paid for the use of whatever song is being used.
Unless you already have connections in the music industry, I suggest starting with a publisher that is established in the business. Usually they have established relationships that have taken them years to establish. Splitting your royalties is a fair trade off when you consider how helpful they can be in getting your music into the right hands.
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