It matters because it is the closest single measure of whether your service desk is efficient or merely busy. A ticket resolved once costs a fraction of one that bounces between people, and the client's experience of the two is not comparable.
The common failure is optimising it directly. Push the number and technicians learn to close tickets that are not actually resolved, so the issue returns as a new ticket, the metric improves, and service quietly gets worse. It is a diagnostic to watch, not a target to hit.