From what I've read, unless aimed by the player, guns target the hull of enemy tanks as a default. I my experience this often leads to situations where the enemy can see and shoot at your tank's turret while it won't fire back since it can't see the enemy hull (middle part). I've noticed that by targeting turrets you'll almost never end up in that situation.
Furthermore, since it is usually easy to disable tanks' guns in ToW, it has the additional benefit of taking care of those monster tanks your opponent sometimes shows up in. I find it's by far the most effective way of stopping Tiger's/JS2's etc. Just knock out their guns and then finish them off at your leisure.
Also, although I haven't really tested it enough to say for sure, putting tanks in "Defend" mode seems to make them try to find the best possible position (including hull down) in the area they're currently occupying. I'm sure someone out there with more experience can better describe how that's working.