Where Search Engine Optimisation is concerned too many businesses still believe that it is the “Silver Bullet” of online marketing and that once the web site has been optimized the job is done. However, just optimizing your site pages with context, keywords etc is not enough; these are internal factors you can directly control however there are at least a couple of external factors that require constant, long term attention :

Search Engine Algorithm Updates

It is getting harder to maintain a top 10 listing in the search engine results pages (SERPs). This is because the search engines are forever tweaking their algorithms, releasing a few hundred updates every year. With every new update, the chances are that the formula you use for achieving a good ranking for your website will need tweaking too.
And because rankings are relative whichever of your competition understands the changes and is able to update their mix accordingly is likely to achieve a better ranking post-update.

The Rise and Rise of Social

With the launch of Google+, the various new features Facebook releases every year and the waxing/waning popularity of the numerous social networks, the quality of inbound links, one of the prime ingredients of SEO, changes regularly. Every change in the social landscape and the amount of social activity that generates traffic has an impact on the ranking of your site.

Both of these factors are in constant flux and as such they require someone with knowledge and experience to keep up with them; This makes SEO an important part of your marketing mix.