Why developers download Xcode.xip file?
Inside the xip file was just Xcode.app. Developers(in fact anybody) can download the xip file from Apple Developer Website. Downloading from there is the same as downloading from the App Store. Probably is because if App Store is slow you can download from alternative(or some other reason). If you have a paid developer account, you can download a beta version of Xcode by downloading the xip file, which is not available from the App Store.