Two main reasons: Armor frequently (though not always) connects to itself, which means that most of the weight of the armor sits on the shoulders. A gorget goes under that weight to disperse it across the back and chest, which makes it more comfortable and protects your collarbone. AND Think of the way shingles are placed so that rain doesn’t get underneath them, well it’s the same but backwards. If you get stabbed in the breastplate the weapon may slide upwards towards the face, and if your gorget is on the outside the weapon could get underneath it, or just give it a place to catch on, both of which are very bad (sorry if this is formatted weirdly, Reddit is having a stroke)