以下c.Args（）> 0的用途是什么

This code is from the cli Go package: https://github.com/codegangsta/cli

package main

import (
    "github.com/codegangsta/cli"
    "os"
)

func main() {
    app := cli.NewApp()
    app.Name = "greet"
    app.Usage = "fight the loneliness!"
    app.Flags = []cli.Flag{
        cli.StringFlag{
            Name:  "lang, l",
            Value: "english",
            Usage: "language for the greeting",
        },
    }

    app.Action = func(c *cli.Context) {
        name := "someone"
        if len(c.Args()) > 0 {
            name = c.Args()[0]
        }
        if c.String("lang") == "spanish" {
            println("Hola", name)
        } else {
            println("Hello", name)
        }
    }

    app.Run(os.Args)
}

I'm a Go beginner and I understand everything, except this part:

if len(c.Args()) > 0 {
    name = c.Args()[0]
}

What does that block says? Why is it necessary?

The function Args returns an object Args which is a slice of strings (see context.go):

type Args []string

To get the first element of that slice ([0]) one has to check beforehand if it's not empty, thus the len test. If you don't do this and the slice happens to be empty, you get an index out of range runtime error and your program panics.