Many plugin.rb files initialize a Rails Engine, which adds various things to the load path.
When we don’t have a rails engine, you’ll see lots of require_relative calls at the top of the plugin.rb file to compensate.
I think this is because the instance_eval is executing the plugin.rb file. If the plugin.rb is not executed, then the rails engine is never created, and the extra paths don’t added to the load_path.