Allaah the Most High said, regarding those who learn and practice magic:

وَلَقَدْ عَلِمُواْ لَمَنِ اشْتَرَاهُ مَا لَهُ فِي الآخِرَةِ مِنْ خَلاَقٍ

And indeed they knew that the buyers of it (magic) would have no share in the Hereafter. (al-Baqarah 2:102)

And the "buying, purchasing" mentioned in the verse, Shaykh Saalih Aal ash-Shaykh says in explanation of the verse, as occurs in his explanation of Kitab ut-Tawhid on the chapter on magic:

And the magician he has purchased the learning of magic, he has purchased, purchased what thing? He has purchased magic in place of what thing? In place of Tawhid, so the price is Tawhid, the price is faith in Allaah alone, and the object (being bought) is magic... so it means the one who gave his religion as consideration (something of value) for that thing which he took, which is magic...

Since magic relies upon the agency of the Jinn (see here), then a man or woman does not become a magician up until the Jinn can verify that this person is eager to become one truthfully and practically, and that he desires to harm the people, and thus when an individual first starts making contact with the Jinn, they will require from him or her the following, as examples:

• To disbelieve in Allaah, through reviling Him, or transgressing against Him (such as in the manner described below)
• To abandon the prayer.
• To pray in the toilet area.
• To put the mus.haf (copy of the Qur'an) under his feet and to enter the toilet with it area, as a means of defiling and belittling it.
• To write the Qur'an with impurities such as menstrual blood or faeces as a means of belittling it.
• To commit incest.
• To remain alone in a dark room for forty days without coming into contact with water.
• To remain for a long time in janaabah (state of sexual impurity) without bathing.
• To remain in a dark room, or some deserted isolated place where the rebellious devils amongst the Jinn come and perform lewd acts with him or her.

Shaykh Saalih Aal ash-Shaykh also says in the same aforementioned chapter:

Magic in all of its types contains the use of the devils and seeking aid from them. The devils do not serve except the one who seeks closeness to them. You seek closeness to them with what thing? With sacrifice (of something to them). He seeks closeness to them with what thing? With seeking aid. He seeks nearness to them through seeking refuge (in them) and so on. Meaning that he directs some of the worship (that is only due to Allaah alone) to them. And I have looked in some of the books of magic and I found that the magician - in accordance with what that autohr described - that he cannot arrive at true, real magic and the Jinn will not serve him as is desired until he belittles the Qur'an, or belittles the copy of the Quran (mus.haf) or until he disbelieves in Allaah, the Mighty and Majestic, and His Prophet (sallallaahu alayhi wasallam).

The devils love disobedience in Allaah and kufr and shirk, and thus, they will only serve and carry out the requests of people who are prepared to do acts similar to those above. If you recall, Iblis refused to bow to Adam (alayhis salaam), and thereafter beguiled Adam to disobey his Lord. Thus, enmity was set up between Iblis and his offspring and Adam and his offspring, and Iblis knows that his destination is the fire and that he has simply been given respite for a period. And thus he is using that period of respite in order to misguide and lead as much of Adam's offspring into the fire with him. The Jinn in general are characterized with envy, hatred, jealousy and mischief in general, and they detest revelation and prophethood, which is why they request the aforementioned acts of desecration from those seeking their aid in witchcraft. And the relationship between these two categories of people is mentioned in the verse:

وَيَوْمَ يِحْشُرُهُمْ جَمِيعًا يَا مَعْشَرَ الْجِنِّ قَدِ اسْتَكْثَرْتُم مِّنَ الإِنسِ وَقَالَ أَوْلِيَآؤُهُم مِّنَ الإِنسِ رَبَّنَا اسْتَمْتَعَ بَعْضُنَا بِبَعْضٍ وَبَلَغْنَا أَجَلَنَا الَّذِيَ أَجَّلْتَ لَنَا قَالَ النَّارُ مَثْوَاكُمْ خَالِدِينَ فِيهَا إِلاَّ مَا شَاء اللّهُ إِنَّ رَبَّكَ حَكِيمٌ عَليمٌ

And on the Day when He will gather them (all) together (and say): "O you assembly of jinns! Many did you mislead of men," and their allies amongst men will say: "Our Lord! We benefited one from the other, but now we have reached our appointed term which You did appoint for us." He will say: "The Fire be your dwelling place, you will dwell therein forever, except as Allah may will. Certainly your Lord is All-Wise, All-Knowing." (Al-An'am 6:128)

Ibn Taymiyyah comments on this verse (an-Nubuwwaat, pp, 311-312, through Fath ul-Mannaan, a compilation of the writings of Ibn Taymiyyah on the Jinn):

As for magic and soothsaying, then it is from the assistance of the devils to the sons of Adam, for the Jinn inform the soothsayer (with information) and likewise the magician, he kills or brings about illness or levitates in the sky through the devil's assistance of him. And so their affairs depart from what human's consider the norm by way of the devil's assistance of them ...

Then he quotes the above verse, then he continues:

For the Jinn and humans, they sought benefit (utility, pleasure), some of them from others, so these ones used those ones, and those ones used these ones in many affairs. All of them did for another whatever his objective was so as to aid in his objective, and magic and soothsaying (fortune-telling) is from this angle.

And thus, the Jinn seek their benefit through the magicians which is the desecration of the sanctity of revelation, prophethood and the religion, and the magicians seek the utility of the Jinn in bringing about or repelling harm, or seeking fame, fortune and the likes.