It matters because it is the number the client experiences. Internal effort is invisible to them; elapsed time is not, and a ticket that sat waiting on a vendor for two days is two days of their problem regardless of how little of it was yours.
The common distortion is measuring it against tickets that were closed rather than tickets that were raised. Excluding the ones still open flatters the average precisely when things are going badly, which is when the number was supposed to tell you something.