1. The most important reasons to carry out SEO and keep your web site pages updated is because your website design may be faulty! In the past, web designers stuffed in keywords, which most search engines found and indexed. Today, the rules keep changing. Keyword stuffing is now actually penalized by the search engines. You need to bring your web site up-to-date w.r.t. the search engines requirements.
2. The search engines constantly change the rules and web sites need to keep abreast of these changes and follow the rules to get good rankings. For example, Google started insisting recently, that all websites include a site map in an XML format or use an alternate specialised format which will make it easier for them to find / index your site. However makes it more difficult for the average small or medium sized company to do in-house and it needs to outsource the task to a SEO expert.
3. Web site rankings change all the time. A site that was once ranked high may suddenly lose ranking. Reasons for this need to be ascertained and the web page tweaked to restore the earlier rating.
4. Your competitors are awakening to the potential of the Internet. All these years they depended on their own sales team and sales effort and on participation in the right exhibitions and fairs to boost sales. They now suddenly realize that there are over a billion customers on the net and it is very important to attract their attention and create awareness about their products / services and make sales online. Online sales is suddenly becoming a very important component of the overall marketing mix.
5. Selling online has become safe and easy. Receiving money via the Internet, likewise, has also become safe and easy.
6. The way customers search for information is changing rapidly. It is now easier to get information from the net than from a source such as the Yellow Pages or a Trade Directory. Customers are, consequently, seeking Net-based information rather than from printed sources. Consequently, your company website needs to be found quickly by potential customers.
7. If you have high rankings on the Internet, a competitor may hire a "Black Hat" SEO company to destroy your high rankings. This is not unheard of these days. (Black Hat SEO is unethical SEO practice that can fool search engines into penalizing your company's website for perceived illegal activities).  A competent SEO company like Spark Consultants can identify this and offer solutions that can help you regain past listings.

Next: What it will cost : How can my company benefit from SEO