I ran across some code that adds an extension to this struct having the Identifiable protocol.

Why not just do this instead?

struct SomeItem: Decodable, Identifiable {
...
}

import Foundation

struct SomeItem: Decodable {
    let id: Int
    let url: URL

    private enum CodingKeys: String, CodingKey {
        case id
        case url = "webURL"
    }
}

extension SomeItem: Identifiable { }

You could do that.

A reason not to is code organization. Keeping all the stuff for conforming to a particular protocol in its own extension makes things just a little bit neater.