Purpouse of classes directory in a Plugin zip file

I am writing a simple plugin, I set up the environment as suggested in plugin-template-master, put my groovy code and compiled the plugin.
Eventually I took a look at the zip file, there is a directory 'classes' containing a set of groovy code, unneeded by my groovy code. I noticed that also non-custom plugin have these classes, whose content varies from plugin to plugin. What is the purpouse of this directory ? How groovy script are selected and put into it ?

Thank you for your help.
Evelino Bomitali