What is the significance of the defensive war of attrition?

Its significant because both sides tryed to use it and "wear out" the other side.

A defensive war of attrition is a strategy typically chosen by a side which does not have sufficient military power to adopt an aggressive posture. That is, the side lacks sufficient military might to effectively attack their opponent.

Rather, the strategy chosen makes use of the myriad advantages that a defensive position enjoys (choice of terrain, maximal use of that terrain, shorter logistical supply, etc.) in an attempt to reduce their opponent(s) military advantage by forcing the other side into costly battles.

The significance is that a defensive war of attrition *must* be fought on one's own territory, so any collateral damage will affect the defensive side much more heavily. This means the toll on civilians and infrastructure falls almost exclusively on the side choosing this strategy. It also requires that the war be a rather long one, as a key component of a war of attrition is that it requires considerable amounts of combat to slowly reduce the other side's military force. However, it does give the side choosing this strategy a reasonable chance against a significantly more powerful opponent, and also significantly improves the chances for a favorable negotiated settlement of the conflict.