system
Given the following post, respond as if you're commenting on Reddit:
human
[Supernatural]Why do the angels carry those blades?

Why do the angels carry blades that can kill them?  Prior to events in the show, there has never been a reason for them to kill each other and they express shock that an angel even can be killed.  Why carry blades that can be taken from them and pose a danger to them and their kin?  To kill other things?  Weaker blades that can kill anything but don't even phase angels exist.  To fight Lucifer?  Only Michael was expected to do that, and the "sword" there wasn't literal.  Why does every single angel need to carry one?
gpt
Because in the world of Supernatural, this makes them a "nuclear power", so to speak. You see how terrified demons and monsters are when they hear about Angels - it's because the weapons of heaven can kill literally anything. (Minus a couple of specific people.) You don't fuck with a group who can all casually dust any evil lord you ever met. 

As for why they aren't designed to not hurt Angels, well, occasionally they need to be used for that. Angels go rogue, or need to be taught discipline. Also, they're big on honour, so if you get stabbed with your own blade, your fellow Angels will probably just think you deserved it.