It is used as part of Qt's checks that the actual plugin dynamic library exported interface and the interface that the program is expecting to see are the same. If your program has a series of versions over time and the plugin interface changes then this identifier should change. Clearly it is good if the identifier you use is not the same as the identifier used by anyone else so you cannot accidentally load an incompatible plugin and crash your program.

Other checks that Qt does are checks of compiler compatibility between you application and your plugin.