I saw a post on this topic on an online discussion forum. The author proposed that if a site has a small universe of users, ie an internal site for a university, church, or business customers, and it is not intended as a tool to market anything or increase sales, then SEO is not necessary.
I've seen many instances of companies with similar names that could result in an Internet user not being able to find the company he does business with. For example, if XYZcompany.com is already taken as a URL, a company would use some sort of variation -- either a different extension, .net or .info, for example, or alter the domain slightly, like XYZcompanyinc.com.
If there are multiple XZY Companies across the country and the site's not search engine optimized, it's possible customers would not find it even under the company name. Not only is that possible, we've seen it happen.
My belief is that if a company determines that a web site is important for their business, whether for sales or customer service, then they make an assumption that people will find it and go to it. In this increasingly competitive web environment, the best way to ensure traffic is to optimize the web site.
Even in the case where a site is only targeted to a well-defined group, you can't make the assumption that everyone in the group will retain the business card, print ad, direct mail piece or letter that had your URL. You'll get a segment of that population that wants to go to Google, type in your name and click on your site.
Why make it difficult for them? Read more about "What is Search Engine Optimization?"