When and why to return an interface in Golang?

Example code:

type IClient interface {
    UploadFile(sourcePath, host string) error
    CopyFile(sourcePath, destPath string) error
    DeleteFile(sourcePath, option string) error
    GetChecksum(sourcePath string) (*string, error)
    GetSize(sourcePath string) (*float32, error)
}

type FileWorker struct {
    Extension string
    Host string
}

func NewFileWorker(fileExtension, host string) IClient {
    var fileWorker = &FileWorker {
        Extension: extension,
        Host: host,
    }
    return fileWorker
}

I have seen other "New" (or "Get") function returning interface like above. But it does not actually return the interface, but instead a struct that implements said interface. In turn, that struct type is the receiver of various methods implementing said interface.

Question: When and why do we return interfaces? Currently I see this is a roundabout way of doing things, resulting in one having to trace back through multiple files while trying to understand some codes.