Tags are simply names assigned to particular object. They do not carry a meaning or action, unless there is a specific script that does something to objects having a particular tag.

I know that campaign missions use 'hidden', 'enemy' and other stuff. However, you don't have to use any of it since logic dealing with these tags most likely will not exist unless you copy it.

Hence, the question: why do you need to use tags?

Some tags do have meaning and an action. Like the two I mentioned above. I'm just trying to see what tags are out there that most people use when they are making a mission or a map. For example: I'd like to know if there is a tag that can be added to buildings to keep them from being destroyed.

There are two levels of tags that you can apply: entity and map units. Entity tags are coded from environment interactions, hence are somewhat hardcoded (though can be changed or amended even). Map unit tags are used on maps to identify logic associated with particular map scenario.

To answer your question. Yes, you can clone an entity, add indestructible tag (this requires digging) and use such entity in map. However, it will be a new type of object that would be strictly limited to your own mod. It is possible to edit entity without cloning, however that would affect all other mods (and maps) which is not desirable.

To expand more. Try opening properties file for any weapons, or bullets (ammo that is used for particular weapon in game). There are tags associated with it. You can then try to locate buildings to find one you need and experiment with adding new tags. From memory, you can take a look at those flag boxes entity and see what tags it has, since it is indestructible. Then once you clone entity (use new name for .def file), you can add newly tags.