The cage is supposed to detonate the heat round (which both the rpg and javelin have to some extent) prior to impacting the actual armor. There’s usually a shaped charge in the round and a stand off with the primer in it that ignites the round at the convergence point of the shaped charge. If you set that off prior to the round hitting the armor it’ll just splatter against the armor rather than penetrate it. Some weapons have a tandem charge, that has two shaped charges to help it get through ERA or I imagine cope cages.