Navigate to "migration" and then click retrieve.
Enter changeset name. If you want to download package.xml for different changeset, then enter names seperated by comma.
Click on "next" button and then retrieve button.
Download zip file and after extracting the files, you will get complete metadata and package.xml.
Benefit of generating package.xml
Before deploying changeset to target org, you can extract package.xml from changeset. Now you can utilize this package.xml to extract metadta from target org before deployment which will work as backup for target org metadata.
Suppose you have to rollback after actual deployment, then you can utilize backup taken using package.xml to redeploy the old metadata in production.
Hope this will help!!