As @Akashiel already mentioned, enns originate from the religious practice of demonolatry, and it should be noted that not all demons have them (as far as I have seen, enns are traditionally used for the spirits of the Goetia; demons that originate elsewhere either do not have any, or have never bothered to provide them to their summoners). Some modern magicians have asked the demons without enns to provide them so if you search this forum, you will find some new ones people have come up with.
Enns are never necessary or mandatory. You can easily evoke a demon without them.