In the Java context, one reason why JSON might want to be used, is that it provides a very good alternative to Java's Serializationserialization framework, which has been shown (historically) to be subject to some fairly serious vulnerabilities.
Joshua BlochJoshua Bloch discusses this in depth in Item 85 "Prefer Alternatives to Java Serialization" (Effective JavaEffective Java, 3rd Edition).
Java's Serializationserialization was initially meant to translate data structures into a format that could be easily transmitted or stored. JSON meets this requirement, without the serious exploits referred to above.