yeah you see an all powerful being/a being without struggle (i.e. your proposed humans without the devil concept) lacks purpose. this being has everything it needs already. he has no need for anything because he is perfect and without discontent. he wants nothing because to want something requires that you are lacking in some way. that you are discontent; something that a being who is not challenged by struggle never experiences. a flawed being (like say, us) is constantly surrounded by and filled with discontent. we don't have this, we don't have that. we must somehow try to find some way to achieve out our desires. this fills up our lives. think of how often you do this from the simplest things to the most critical things. You're hungry so you must find a way to get food. you have sexual desires so you must find a way to attract sexual partners. and in general you must find a way to avoid all the dangers that threaten your fragile existence. To top it all off, you must also try to find things that can make you happy throughout this life. all of this is a struggle in some way. from one struggle to the next, from the littlest of things to the largest. to remove struggle would in one fell swoop remove our purpose. we would become those devil-less humans you speak of. beings that never have to struggle or know what it is to be faced by something that challenges our happiness. and in that same strike we would invalidate ourselves.