Something that pulls metal from that far away combined with a medieval/renaissance society? Torture, Prison, and maybe Hell come to mind.
As a gross over-simplification, in societies of that era especially the early end: magic was still a possibility, religion was a given, and science was quasi-heretical if you didn't agree with the local religion or offended someone important.
Torture
Remember the Rack? Arms tied at one end, legs at the other, and the victim was slowly stretched until something gave way. With a strong magnetic field of this nature, you simply need to anchor the person at one end, and attach some metal to the other end and let them hang. Sideways. Bonus points if you provide feathers.
Prison
Just encase them in metal, or give them metal boots and gloves and let them go. They won't be coming back. Think of a long walk off a short pier with concrete boots... only with metal and magnets!
Hell
Given the strength of that field, you get close enough, it might be able to pull the metal out of one's body, such as blood. Should that be the case, then you have a good candidate for the "Yellow Springs" to borrow an Asian metaphor for Hell. ItAnything which could slay a person invisibly by sucking the blood out of one would very likely give rise to the notion of demons and Hell, if they didn't already have the concept from mythology and/or religion.
As an added bonus, it would definitely interfere with birds and other creatures which rely upon geo-magnetism.
As another thought, once they discovered electricity, and the relationship between it and magnetism, you have the makings of an almost perpetual motion engine core.