They're not. You can discuss a topic without posting a request for changes.
As examples, see this and this topic, both of which aren't suggestions or requests.
OTOH, the request and suggestion tags allow users to quickly search for topics that are about these specific and rather common issues.
One of these is a verb-phrase, a process, while the other is a noun, a specific thing: Therefore, they are distinct.
There may be some overlap, as the process answering-questions results in the specific thing answers, but the process is not the thing itself, merely the cumulative actions by which the thing comes about, these are different tags, have their ...
Could a ♦ please act on this?
Nobody came forward in the defense of this tag in over a month
We already have rather well received duplicates for this topic
Perfect +9/-0 score to the burn request
What more do we need?
It doesn't really need to be implemented.
The only users this affects are those with less than 300 reputation, and if they feel that their question needs a tag then they should first ask their question and apply whatever tags that are relevant and currently exist. They would then come to meta and request that their question is re-tagged with an extra tag ...