I can't speak for Jyp, but from my experience and from what I've read from other devs it's any of the following reasons:
- There's additional details of the world they want to share or establish but can't easily with their main title so it's done in a side title
- Had an idea that they wanted to explore
- Feeling a little bored or burnt out with the main title but still wanting to work on something
Again, I can't speak for what Jyp's reason(s) is, but it's usually one of those. Especially as the title starts to get older and you learn more and more about the tools you're using along the way.