Why two types of relationships links?

Relationships links may be either “self” or “related” as described here. I’ve read the explanations but I still don’t understand why there is a need for two links, and what the difference is. Also, by principle, shouldn’t a relationship be handled as any resource representation with one url, and then the verb would be used to instruct the action?
Changes to relationships via such links, are they limited to create/delete of the linkage only (not creating/deleting the related resource itself)?
Example:
“links”: {
“self”: “… /articles/1/relationships/author”,
“related”: “… /articles/1/author”
}

this thread might help?