A few things that I have noticed.... Defense isn't just for "Help Defend" alerts. 1. If you defense is lower then your food and water combined, you are more likely to get attacked. (24 food 40 water = 64 resource and only 30 def will result in more attacks) 2. If you fail to defend a settlement, the damage done is based on your defense value ratio. (Few defenses and all generators, water filters, and food will need to be repaired - Lots of def and you will not need to repair as much. I think it also determines how many settlers are killed, but I haven't confirmed this) 3. Enemies spawn if you are in range of the settlement. If you do not go, it's all just number rolls or RNG. You only have to be nearby for the spawn to trigger. I was at Red Rocket and the attack started at Sancuary. And a comment as to why do you need a lot of defenses even though you have to help defend? Well there are other world events taking place. Not sure if it was a bug, but a caravan was being chased by Synths and they fled to my settlement and a shootout took place between my settlement and the synths. So I was happy we helped out the Caravan.