Files are all over the Internet, and Go should provide a reasonable abstraction in its stdlib to handle this.
Files are everywhere. Local disk is the obvious place. But they're also in storage buckets like Google Cloud Storage and Amazon S3. They're in code repositories like GitHub and Google Source. They're in personal storage like Dropbox and Evernote. They're on other machines or filesystems on corporate Intranets. Tests which write to disk are much less error-prone if they write to volatile memory, such as a RAMFS.
Go 1's os
package provides an os.File
struct. The implicit assumption is that local disk storage is the only thing the stdlib should care about, and that external packages should provide access to these other sources. In 2007, this would have been a reasonable approach. In 2017, files are increasingly unlikely to be local.