It is used when you are using PMD to analyze your Java Source code. After build is complete, it will help you display warnings on Dashboard. We use it when we develop webdriver test scripts using Java.
This is used when you are using Checkstyle to define coding standards. Plug in helps you in scanning related XML files and display result on dashboard.
Cobertura is used for checking code coverage. Cobertura plugin will help you in displaying report from Cobertura.
Hope it helps