I actually had to open another question with "opposite" in its title to figure out what tag to use for my What's the opposite of "server"/"server operating system"? question. I think it would be a valuable addition to the site if there was a tag synonym opposite for the existing tag antonym.
When a tag is made a synonym of another one, and users enter the first tag, it will be replaced with the second one. For the system is like the first tag doesn't exist at all.
Adding a synonym of a tag is done when there are users who use two different tags, but both the tags have the same meaning, or when users write a tag using the wrong spelling. It could also be used when questions are migrated from another site that uses tags different from the tags already in use in the site where the question is migrated, and there is a correspondence between the tags; in this case, the synonym is created when there enough migrated questions that use that tag.
In other cases, it doesn't make sense to create a synonym, as (apart the cases I have already reported) it doesn't add any value.