The basic traditional Jewish understanding of the purpose of human existence in this world is for us (as individuals and as a group) to bring ourselves closer to perfection/God through our own efforts. This means, of course, that we must begin at some "distance" from perfection/God, and that there must be some degree of resistance towards movement in that direction.
Being that perfection/God is inherently the most desirable goal in existence, then if we were fully capable of perceiving this goal there would be no true "choice" in pursuing it. It is therefore necessary for it/Him to be "hidden" to some degree from our perception, so that we must make a choice between the goal of perfection/God and the satisfaction of other drives. This "hiddenness" must be carefully balanced so that there remains enough knowledge of perfection/God that we can (and, in fact, are morally obligated to) rationally recognize that it is the proper goal of our existence, but not so obvious that that its inherent desirability completely overwhelms our desire to satisfy our other drives.
These other drives, which include all the natural desires of human existence, ranging from basic animal urges (e.g. food, sexual pleasure) to "higher" human drives (e.g. intellectual curiosity, aesthetic pleasure, creativity, even the desire for spirituallity), are the essence of what we refer to when we speak of the yetzer hara (Evil Inclination) or the Satan.
Nevertheless, while these other drives do serve to provide alternate desires/goals that can distract us from our primary purpose (moving towards perfection/God), this does not mean that this is the only reason, or even the primary reason, for their existence. Every natural urge and desire has an important primary function in of itself, independent of its secondary function as a spiritual impediment/distraction. The Jewish ideal is not to eliminate these drives, but to control and channel them in spiritually productive ways. No human drive is inherently evil; the evil is only in seeing the satisfaction of these drives as an end in itself instead of a means towards the true goal of moving closer to perfection/God.