EA occasionally offers this service and has done so with Satan, Lucifer and Belial. Essentially, you tell the magician what you want and the magician relays it to the demon, and who then relays back the demon’s wants. Once an agreement is reached, the magician writes it up, and has the demon sign it, and then mails the written pact to the client. The pact does not take effect until the client signs it.
The benefit is, you get to reap the rewards of having a powerful spirit working on your behalf, even if you are incapable of evoking yourself for whatever reason.
It is difficult to know if a spirit accepts a pact if you can’t hear what they say. Divination can tell you whether a pact was accepted or not, but it can’t really tell you why it might not have been. That’s where a third party can be helpful.