Java Archive (JAR) plug-ins contain Java code that you can call from C# scripts. They are useful if you want to interact with the Android operating system, or just call Java code from C#.
This type of plug-in is useful if you plan to reuse Java code in multiple projects, or distribute it to other people. If instead you only want to write a small amount of Java code for a single project, then a Java or Kotlin source code plug-in might be more appropriate.