Giving sacrifices to the spirits puts you at their disposal. The sacrifice can be anything - livestock or dead meat, incense, libations, even services - but no matter what you give, it allows the spirit to take the advantage. It is showing the spirit that you are willing to meet its needs before you expect it to satisfy yours, which is the substance of demonolatry.
The spirit who receives the sacrifice may agree to do as you say, then fail to do anything at all once given license to depart. It may fulfill your wishes in a way that is contrary to your original intentions; or it may do exactly as you request but in such a way as to benefit itself, turning your goals to its gains. None of these are necessarily harmful or dangerous, but neither are they the marks of a "powerful" magician.
The two kings with two attendants require "Sacrifices, Offerings, and Gifts," presumably for their servants. There are differences between these three things, though I can only speculate on what the differences might be. It should be enough for Paimon to offer beer and bread, but for Belial I could not say.
None of the spirits should trusted about the least thing until they have proven themselves to your complete satisfaction. Do not believe they will (or can) do what they say, and do not believe what they tell you has happened or will happen, or anything they teach about any subject whatsoever. Make them prove it, and be ready to recall them, bind them, and torture them with the curses if they do not make good on promises or if it looks like they were full of crap.
You might wonder, "Why bother, then?" and that is a well-warranted question that deserves some thought before you act. It is probably good to provide yourself with a ood answer before attempting any sort of magic, especially with these spirits who are everywhere described as wicked and stubborn.
As a matter of perspective, though, it is common to deal with all sorts of people who are are not trustworthy yet who are needed in a professional capacity... ever buy a used car? I think of it in the same sense... you know the guy is going to try to rip you off in one way or another, especially if you "give in" on something or another, and it's best to recognize the fact and deal with him accordingly.
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