User blog comment:IamJakuhoRaikoben/Chapter 445: The Ugly Fairy, Review./@comment-26451407-20150720022152/@comment-7418318-20150726150355

Someone who was threatened by an enemy who's magic was much stronger than their own would be my guess.

They could have been anti-Dragon weapons, anti-Dragon Slayer weapons, a last-ditch defense against Alvarez, anti-Zeref weapon or something similar.

The better question to me is, who had the knowledge and the ressources to invent such things and then build them all over the entire continent? And how was it ever tested? Did they build a much smaller prototype and then just scaled the thing up?