This may sound like a silly question but when a title's TPB or an omnibus is being put together, initially collecting the entire run of a title and sell them as one whole book, what is being considered when collecting issues from other related titles?

Like for example, hypothetically: Marvel decides to collect all 100 issues of the The Avengers written by critically acclaimed writer *Marvelo Marvelson into one giant book they'll sell in stores across the universe. There's a paragraph behind the book where it says that it is collecing not just Marvelson's Avengers 1-100, but also Issues 45-50 of Amazing Spider-Man by *Aranha Hombre-Maldicion, Issues 60-63 of Iron Man by Zinc Calcite, and The Fantastic Four Issues 4-9 by *Quattre Van Buren.

What is being considered when the publisher chooses these particular set of issues from non-Avengers books to be included in the collection? Are the stories featured very relevant to the main content of the book? Where they part of an event as tie-ins which makes them too important to skip? Would certain dynamics and situations seen in the Avengers be better explained if they include the issues from non-Avengers books for the benefit of readers?

And it does feel very overwhelming when you come across these kinds of books and see which comics they were sourced from. A lot of these collected editions all look big and thick with so many issues placed inside them.

Thank you. :)
(*all hypothetical names)