{Horned Helm} - what's the difference between equip and attach? And when can I pay two green to attach?
The equip ability attaches, but can be activated at sorcery speed only. The {G}{G} ability is a regular activated ability, meaning you can move the equipment around at instant speed.
Quick example of when you'd want to do that: someone casts {Eaten by Spiders} on your equipped creature. You can pay {G}{G} to save your equipment (if you have another creature to attach it to).