Slaves for Power

Sorcery is another name for magic that is 'taught' by demons. In truth, it is not so much taught as it is granted, for the sorcerer does not attain mastery or understanding, but rather opens up their souls in order to channel power through the demon and into the world in order to work magic.
In some ways, the tie between Sorcerer and Demon is similar to that of soul pacts wherein the demon takes the soul of a human being and uses that soul to grant wishes. After the Sorcerer finds or calls the demon, they barter with the demon for the powers they seek. However, unless the Sorcerer is knowledgeable about the arts of magic, it is likely that they will barter their souls away cheaply. The Sorcerer can bargain for specifics, but the demon selects the techniques and sources which the Sorcerer/slave has access to.
But a slow, deeply satisfied smile came over him, and his breath quickened. 'So softly it starts,' he whispered. 'Foolishly clever and with an unsurvivable trust. It just saved your miserable life, that questionable show of thought, my itchy-witch.' Al’s smile shifted, becoming lighter. 'And now you will live to possibly regret it." ~Kim Harrison, The Outlaw Demon Wails
Why Sorcery?

Some people want power without restraint and do not want to go through the arduous steps to acquire it. Some people do not want the discipline that goes along with mastery of magic and self. It is commonly said that "by the time you can kill someone with your pinky, you've also acquired the understanding that you do not need to". Sorcerers defy that concept, throwing caution and discipline to the wind in order to wrap their hands around raw power. That it comes at the cost of their souls is seen of little consequence.
Because sorcery is obtained at the cost of one's soul, many people view sorcerer's as evil. Yet, humans are complex and have a variety of motivations. Some may be seeking revenge for some wrongdoing, others may be tired of being inconsequential, and some may be trading their lives for the life of someone they love. Regardless of why a person chooses to become a Sorcerer, they are free to carry out whatever goals motivate them, as long as these plans do not directly interfere with the Demon's own and the Demon has not forbidden a particular course of action.
The power of a Sorcerer
Sorcerers gain access to all the same techniques that a Magus has access to and uses them with the same strength and skill of a Master Mage. Regardless of the techniques they know or employ, a Sorcerer is considered to be at a Master level. This incredible strength is one of the reasons why people who desire power barter with demons in the first place, hoping to achieve immediate skill without effort.
A sorcerer can choose a maximum of 3 Sources, and 6 Techniques, although these abilities are used with the strength of a Master.
The Price of Sorcery
In obtaining the power they seek, the Sorcerer forfeits their souls. To some people this is considered a fair trade, and upon death the soul becomes the possession of the demon. They also forfeit their free will. A Sorcerer might have great power, but they cannot defy the letter of their demon master's commands, and the master's merest suggestion is treated as an ironclad command.

Unlike demonic thralls, Sorcerers do not get the associated skills and benefits of the Seals. Instead, their trade is magic power in exchange for their souls. A Sorcerer can be made a thrall and have seals placed upon them, but this is an arrangement that is not assumed or automatic.
What the Demon gets out of it
Other than a slave and servant which can be commanded perfectly, if not trusted implicitly, the demon can draw on the Sorcerer's soul, using it to empower itself and its own powers and abilities.
As this agreement is contractual, the demon owns the sorcerer's soul through life and after death. Neither the demon nor the sorcerer can break their side of the deal, however, if the demon is killed, the sorcer's soul is returned and the sorcer's magical powers are removed.