Beacon placers tend to prefer the backside of buildings (eg the PP in city) or inside structures. That way they have at least some cover/protection than if they were outside. Also, vehicle/engineer rushes will inevitably end at a building door, and they all pile in, so well-placed mines inside doors will do the business. Not kill all maybe, but damage/scare 'em

Once the beacon is placed, and the whole world is warned, it's far easier to cower inside a structure than in the open, as you tend to become a lead magnet

There's no hard and fast, but times when either method is needed.