That’s simple, the reason why I’m building up sequels is because of answers. Stories can be taken as “finished” when it comes to reaching the conclusion of the story. In Harry potter, for example, in the first book he took his adventure in Hogwarts and met the big bad, Voldemort, in a few minutes. After defeating him, harry went back home for a summer vacation. Yet, it is not the end because of Dobby, who warned harry not to go into Hogwarts, thus that raises the question- a new calling for adventure in the sequel.
Also Rick riordan’s “Heroes of Olympus”, it is basically a new sequel to Percy Jackson- because of a new calling. Making a new calling is necessary when it involves more lore, more conflicts within the characters situation, and how their actions imply to the consequences in the future. Unless a sequel has no point at all when it has so many hated holes in the story, I’m looking at you The last Jedi!
My point is that making a sequel is like a whole package of biography of the characters; how they live and what they had done, that has been stretched out into different pieces to make a story-line to tell a tale from the past, present, and even the future.
Aside from magic, thrilling action, and the enchanted world around them, my characters’ development is very essential for me because they are the ones who grow and how they start their journey due to what they want and need in order to do something. Other side characters exist because it depends how the MC interacts to them- like an rpg game. The question why do you need a sequel is more likely a question why do I need a sequel? Because there are so much going on in their lives that I, as a story-teller, want to tell.