Does “metres gained” exclude clangers (e.g. 50m kicks to an opponent on their own)?
It's one of the worst "buzz stats" going around and the commentators love it.
A 50 m bomb into the forward line gives you +50 even if it ends up coming back out, whereas a 50m switch across the ground that leads to an easy goal is worth 0 (or even negative if it goes slightly backwards!)
Obviously not perfect (and a better stat would be metres gained in continuous possession from your possession, so that the sideways kick you talk about that leads to a goal is counted and the 50m bomb directly to the opponent isn't), but it's a stat with some meaning in a game where territory matters.
Getting it forward then locking it in the forward half until you score matters, so in that context metres gained has some meaning. And it at least gives little value to the chipping around meaninglessly in defence, even if it undervalues the switch that breaks the game open.
There is no perfect set of stats for AFL, but it's getting better, and as long as you understand what the stats mean (and it's not always easy to know, since all of these stats that are quoted so often aren't footnoted with "meaning" information) and what they don't mean they can help with an understanding of how a game was won or lost.
I am pretty sure that a lot of times commentators don't know what the stats mean or don't mean!!