The motivation behind this JEP is laid out in the JEP under the section "Motivation".
As I understand that section (and I wasn't involved in writing this JEP), good and popular marshalling libraries for JSON already exist, and the JEP clearly states that it is not the goal to replace them or perform their role. The JEP says that this package may be what you'd reach for when a program only wants to do some very simple, small tasks with JSON data and the requirements and code size don't merit pulling in a fully-featured JSON library (e.g. when you're writing a one-file script, or exploring in JShell).
As I understand that section (and I wasn't involved in writing this JEP), good and popular marshalling libraries for JSON already exist, and the JEP clearly states that it is not the goal to replace them or perform their role. The JEP says that this package may be what you'd reach for when a program only wants to do some very simple, small tasks with JSON data and the requirements and code size don't merit pulling in a fully-featured JSON library (e.g. when you're writing a one-file script, or exploring in JShell).