if it is a personal website and you only want to share it with friends, then SEO won't be that essential of use...
but for corporate sites, SEO is a "must" thing to do, it can bring in traffic, reputation, popularity for the companies site.

yeah I think it one of the cheapest way to market your site ..

SEO is the basic need for a website by virtue of which you can increase traffic of your site and page rank. SEO is a technique to optimize the web pages in order to make them Search Engine friendly it means you can get higher positions in search results.

i think for a business site yes, SEO is really needed..... because, using SEO to your site, your site's income can increase also making your site popular to the world wide web

It's worth it if you intend the site to generate new business and create awareness. It is not required if the site is only intended to provide information and customers will be given the website url.