"I am no man," He said. "I'm a magician with no magic, and thats no one at all."
As the story goes, Shmendrik was born long ago and apprenticed to become a wizard. His tutor Nikos tried and tried but finally gave up:
My son, your ineptitude is so vast, your incompetence so profound, that I am certain you are inhabited by greater power than I have ever known. Unfortunately, it seems to be working backward at the moment, and even I can find no way to set it right. It must be that you are meant to find your own way to reach your power in time; but frankly, you should live as long as that will take you. Therefore I grant it that you shall not age from this day forth, but will travel the world round and round, eternally inefficient, until at last you come to yourself and know what you are.
Don't thank me, I tremble at your doom.
Shmendrik when he enters into the story, he is working for an old witch who enchants common animals and shows them as mythical creatures.

Throughout the story, Shmendrik does "almost magics", occasionally touching upon the true power of magic. When relating is quest to find magic to Molly Grue, one of the companions of the Unicorn, she asks,

And if you should find magic - what then?
to which he has no answer.

At Hagsgate, the villagers rebuke him as a magician,

Anyone can say he's a magician these days. The old standards have gone, the old values have been abandoned. Besides, a real magician has a beard.

Just before entering the castle of King Hagard, Shmendrik calls upon magic to save the Unicorn from the Red Bull and changes her into a young woman who is named Almathea. He is hired by Hagard, king of the realm of Hagsgate as an entertainer.

In the past, you have performed whatever miracle I required of you, and all it has been to spoil my taste for miracles. No task is too vast for your powers - and yet, when the wonder is achieved, nothing has changed. It must be that great power cannot give me whatever it is I really want. (Hagard to Mabruk)

To defeat the Red Bull, Shmendirck successfully calls upon magic again and changes Almathea back into the Unicorn, thus making himself a true magician. With this came the knowledge of the place of magic in the world.

Then what is magic for? What use is wizardy if it cannot save a unicorn? -- Price Lir to Shmendrik
That's what heroes are for. -- Shmendrik to Lir
Yes, of course. That is exactly what heroes are fore. Wizards make no difference, so they say that nothing does, but heroes are meant to die for unicorns. -- Lir

Through the use of Shmendrik, Peter S. Beagle shows us the constraints of magic and power. Causing miracles to happen on demand takes the wonder out of the world, something we need, lest we become the figurative skeleton (the true talking skeleton in the story has more life than Hagard) of a man that is Hagard, able to move and act but empty inside.

The purpose of magic is to keep the wonder alive, but not cage it as the witch did or as Hagard did with the unicorns.