Why is serum free?

Could you elaborate on why Serum, the decentralized exchange protocol, is often referred to as being "free"? Does this mean there are no transaction fees associated with using Serum, or is it a reference to the fact that it operates on a decentralized network without intermediaries? Are there any potential trade-offs or limitations to Serum's "freeness" that users should be aware of? Additionally, how does Serum's approach to fees and intermediaries compare to traditional centralized exchanges?