Noble Phantasms are a Servant's trump card to pull out whenever they're in the middle of battle. That's why whenever you see someone's weapon, it might not be exactly what you think it is.

[He can't speak for all cases, but he definitely wants to teach her that not to take everything at face value.]

Or it might not even be a weapon at all. It could be anything like a giant ass animal or even a reality bending spell. Whenever it is, it's going to represent that person's identity. That's why they gotta be used sparingly... Not only that, Noble Phantasms consume a lot of mana.