Why have you created a website on the World Wide Web? Is it solely for the staff of your company to see or just your family and friends to see? I am sure there are far more specific motives for the creation of your website.
If it is a business website, then you would want potential customers to find it. If it is an informative website, then too, you would want users to find it easily. Now, the internet is very different from the offline world. In the offline world, advertising and marketing rule the roost.
A quality advertising campaign is one of the mainstays of your business plan. That means, you take your business to customers and they are more than happy to do business with you. But in the online world, it's all about natural results. Think about it. How many times did you click on sponsored results in the last five times that you used a search engine to find something?
Users will find your website naturally using search engines. This means that the traffic that comes to your website will be highly targeted and will convert into business. If you want a continual flow of passive traffic to your website, then search engine optimization is the only way to achieve it.
While other forms of web marketing are expensive and only provide temporary results, search engine optimization(seo) will help you convert your website into a brand name in the global internet marketplace.