I'm not saying you don't need that other information as well, but I like to know the story behind creatures in addition to just their information.
Because these creatures were created by Graz'zt and will obey his wishes and the wishes of his followers IS the answer to this question. It is just as valid an excuse as any other.
I'm not entirely sure what the rest of what you said means.
You would like to know the ‘origin story’ of a monster. That can be useful. But it depends on the setting. Different settings have different origins for a monster or race.
So, buy a Setting Guide for origin stories. A Bestiary is something different.
I don’t use the Planescape Setting. So any origin or explanation that presupposes a Wheel of Outer Planes, or objectively existing gods (or archfey, or archdevils, or demon lords, or primordeals, or any other over-important NPC) cannot work as an origin in the settings that I use. What can work for me is the descriptions of the monsters themselves. The monsters only.
Your post suggests, ‘Because Grazzt’, is an explanation for why Lamia and Jackalwere work together. But it explains nothing. Grazzt could have had, Lamia and Blue Whale, or Lamia and Tulips, or Lamia and Salt Deposits. Why was it, Lamia and Jackalwere? What is it about this pairing that makes sense? The false answer, ‘Because Grazzt’, answers nothing. The description still needs a real reason WHY, HOW, do Lamia and Jackalwere work together?
Your post suggests, ‘Because Grazzt’, explains how the Lamia and the Jackalwere will behave together. Specifically, the post says: ‘Because these creatures were created by Graz'zt and will obey his wishes and the wishes of his followers IS the answer to this question.’ But it answers nothing.
First, ‘Because Robot’ is a terrible motivation for story. Even if Lamia and Jackalwere ‘obey’ the wishes of Grazzt, they are probably not mindless robots. They probably have independent desires. They probably have intelligence, personality, alignment, and a typical behavior and survival strategy. Yet ‘Because Grazzt’, says nothing informative about these vital motives that a DM must know. Moreover, if the monsters have individual volition, how much more does the Human ‘cultist’ have an independent motive! It is irrelevant whether all three are worshiping Grazzt. The monster description would still have to explain in what ways these three creatures might be cooperating together, and what benefit each of them derives from the cooperation.
Saying, ‘Because Grazzt’, is equally informative as saying ‘Because cheese’.
The use of ‘Because Grazzt’ explains nothing. It distracts from and usually prevents the real information about the monster that the DM needs to know to understand how the monster will respond during an encounter. And it scares away customers who dont use the setting that has Grazzt in it.