I saw in a plugin (dfGUI) a HideInInspector attribute in front of some methods. Is that useful? What is the purpose?
Its only for variables, not for methods. And sometimes it is useful, for instance if you have public value made public only because you want to access it from other scripts and seters and geters are not for you. It can save some time if you have many variables in 1 script, and don’t want to spend too much time searching for that one you need in the inspector.
HideInInspector only hides variables so there is no advantage whatsoever in using it with methods.
Are you sure it is in front of an method and not for example an array being initialized with values (uses curly brackets and may look like a method of sorts) instead?