The word "demon" comes from the Greek word daimon. It originally did not mean an "evil" spirit, but simply a spirit intermediate in power between gods and humans. For example, Socrates was quoted by Plato as saying he got some guidance from a daimon. Such spirits could be beneficial, harmful, both, or neither. Only later were they thought to be necessarily "evil" -- that is, "evil" according to what is considered "evil" by the prevailing religion.
As you read through the descriptions of demons in the grimoires, especially the Goetia, you may notice something interesting. Not all the demons are "evil" by secular modern standards. Only some of the demons are said to do nasty things like causing diseases and floods. Others do more beneficial things, like helping people learn various arts and sciences, including math, astronomy, natural science in general, medicine, music, poetry, the ability to write and speak well, and even "ethics" and "moral philosophy,"
If you're from a nonreligious or only moderately religious background, you may be wondering: Why would spirits who teach useful knowledge be considered "evil"?
In hardcore Christianity, the only "good" thing is total subservience to the Christian God. Anything else is "evil," no matter how beneficial to humans.
What's so "evil" about science? It, and the resulting technology, have freed a lot of people from dependence on the Christian God. Before the Enlightenment, quite a few pioneering scientists and inventors were thought of as being in league with the Devil. To this day, Protestant fundamentalists still oppose the science of evolutionary biology, and even some more moderately religious people are still frightened by the science of genetics.
Even "moral philosophy" can be "evil" by hardcore Christian standards, because it means thinking for oneself about morality, rather than blindly following a supposedly absolute set of rules. In the Garden of Eden story, Adam and Eve were forbidden to eat of the "Tree of Knowledge of Good and Evil."
And, of course, hardcore Christians have always been wary of new forms of art and music. A lot of fundamentalists oppose rock music, for example.
To the Satanist, there is no such thing as forbidden knowledge. Aim to "become as gods," to whatever extent that's possible. Knowledge is power.