Superdave: It is an example of a ko threat. Black is ordinarily alive, but if white can play at twice in a row then black is dead. So after the white suicide black must decide between keeping his group alive or taking the ko.

It would be a losing ko threat under territory rules (costing one point) because white is throwing in an extra stone (and black eventually has to capture anyway), but the rule sets that allow suicide are usually area scoring, so it might not be.