Over the last few years searching trends have changed. In the past the majority of searchers used one word queries. Eventually they started realizing they they get better, more accurate, results when you give the search engine a bit more information about what you are looking for.
The more accurate the search phrase you use in your search is, the more accurate the results will be that are returned. Studies have shown that four and five-word phrases often have a higher ROI than one and two word phrases because the searcher is more likely to get results that meet their needs.
The downside of longer phrases is this increases the keyword combination potentials so the number of searches for any one phrase reduces dramatically. This makes optimization more difficult. Instead of optimizing for one general phrase you have to optimize for five very specific phrases. This is the long-tail of keywords, also known as the low hanging fruit. These longer phrases have far less competition and are much easier to get ranked, but also produce lower traffic volumes.
Long-tail phrases should not constitute the primary focus of your optimization efforts. Nor should you focus primarily on short-tail phrases either. A good keyword optimization strategy goes after both simultaneously.