For the most part, you write articles in order to gain someone's attention. When you are writing articles for the internet, that means you will have to gain the attention of a search engine first. Over the years, a number of techniques have emerged to help authors develop their writing so that it is as suitable for human readers as well as search engine crawlers.
One of the most powerful tools at your disposal is keyword density. Basically, you will want to repeat a keyword a certain number of times based on total word length. As an example, a 2% keyword density in a 500 word article would translate to 10 repetitions of the keyword. Depending on the topic this, and higher densities may be easy to achieve.
With some other topics, even a 1% keyword density will look like gross keyword density. In general, anything over 4% is likely to be repugnant to the reader. Unfortunately, some article buyers want as high as 8% keyword density. They do not realize that with that volume of repetition, even the web crawler will stop reading and may well elect not to index the page.
Aside from this, the best articles for SEO purposes will be ones that hit the main target, and also bring in some general ideas. When it comes right down to it, you never know what people will search on the internet for, let alone what phrases they will use to find it. Sometimes an obscure reference can be just the thing to help drive traffic to your site.