They're intended to provide a cool, controlled place to keep the bread away from direct sunlight and most pests, to reduce the rate of moisture loss, and reduce the likelihood of the bread getting moldy.
I'd personally avoid metal in areas with high temperature swings (daily, not yearly) because I'd be concerned with changes in temperature outside causing condensation inside the box. If you have air conditioning and don't have it cycle on/off during the day, this likely isn't an issue.
Some people don't actually like the wooden bread boxes, because the smell of the wood used can transfer to the bread. It's possible that the nature of the wood, if it were unsealed, might help to regulate the humidity of the box (absorb a little if it's too high, give a little back if it's too low), but from what I recall (and it's been more than 15 years since I've used one), all of the ones we had were vented, so I don't know how much this is actually an issue. (it's possible that not all are vented -- in looking online, some of the metal ones claim to be air-tight ... I'm not sure how I feel about that).