Well the goal would be to install the hardware package as it exists in the current development state. Some hardware packages do have the JSON file for Boards Manager installation but that will install a release version of the package rather than the tip. So, for example, for the compilation tests of the MightyCore bundled library examples sketches via Travis CI the first step is to install the Arduino IDE and then copy the contents of the repository to the hardware subfolder of the sketchbook folder. This is known as the manual installation method, as opposed to the Boards Manager installation method. Then the compilations are done not only with the latest versions of the library code but also the latest version of the hardware package.