To contain the energy in the conductor Conductors are insulated to prevent the escape of energy from within them. Without insulators, the conductors will usually release energy into the surrounding substance, unless that substance acts as an insulator. This answer is the result of recollection of a KS3 (high school) science class, and, although quite vague, it is a good outline. It will be useful to search "conductors and insulators" in a search engine, as there will be much more information provided.