It marks the limit of his safe territory for he can be stumped or run out (see Dismissals below) if the wicket is broken while he is "out of his ground".
For he can be stumped. This is what it marks the limit of his safe territory. For he can be run out (see Dismissals below). This is what it marks the limit of his safe territory. The wicket is broken "out of his ground". He is. The importance of the popping crease to the batsman is this.