Ah, you bring up an excellent point:
Keyword stuffing and black hat techniques are bad ideas, certainly.
But keyword density will absolutely help.
The difference I am trying to indicate might make more sense in an example like this:
If you're trying to sell a self-help book, then these are two concepts:
1) You're writing for 'humans'... because you're going to get your article published somewhere where readers are. In that case, you might go with a title like: 'Life a like a bowl of cherries...'
This could bring the readers directly, because you're appealing to them with analogies, concepts, constructs, examples and so forth that draw the relationship to your theme.
But your theme, in this case, is 'self-help'... 'self help book'... 'helping yourself grow'... etc.
Google can't be expected naturally to understand the relationship on your page of 'self help' and 'book' to the following: 'bowl', 'cherries', 'life', or 'like.'
And that is where it matters what you're trying to produce, and how you use wording.
If someone searches Google for...?
self help book and
self help (and you want to first check and see how many folks do search for these, before you bother to write a topic you aim to get organic ranking on), then you will absolutely want to make sure that your natural writing includes those phrases... and instances of those words... and clearly-related variants of those words... as often as you can (density). Yes, write naturally. But use the right wording. Minimize the examples and analogies; put those on the next page, or create a landing page that is meant to be optimized that opens to the next page, where you can then continue with ideas like 'Life is like a Bowl of Cherries' (this being the second page... the one you're not trying to optimize).
When you see cases where perfectly natural writing (non-optimized) is being ranked highly, it's usually because either the writer has struck on a theme that is quite unique in relation to the related searches, and strikes home directly. However, that's extremely tough to find a market for anyway... you might have a page like that as a reference on your site to help your readers see your expertise, etc.
But otherwise, what can drive that page to the top is the amount of traffic coming to it from... self-help blogs, directories, etc. Google counts that kind of traffic, if Google can SEE that take place.
This is why it's very important to use Google Analytics on your webpages, because that is the best way for Google to SEE where your visitors are coming from. If it SEES visitors coming to your 'Life is like a Bowl of Cherries' page from various self-help avenues, then Google can say 'oh, I see... this page has to do with 'self-help.'
And how Google knows THAT is by doing the math on the keyword density and other page checks for those incoming clicks.
Keyword stuffing, by itself, is a bad idea. Even if you get a spike in traffic as the robots first submit your pages, those pages will later be dropped like rocks from the front page of... basically anything.