Most web designers have become web designers after beginning their career by making brochures. They think that a website is a large brochure; they design it and use words as they would on a brochure.
Unfortunately a website isn’t a big brochure and only a good web designer would even know this, let alone explain the difference between a website and a brochure. A website is more like another branch of your business, that's open 24/7, open to everyone in the world with the internet, and let face it, a lot cheaper than having another store, nothing like a brochure which is quite a different medium.

If the website is designed to look pretty or professional without any attention to how the website will be found in search engines, then it will end up just like a commercial that doesn’t get aired. Good SEO is like an advertising campaign except if over the period of the SEO campaign new visitors to you site go from 2000 up to 10,000 new visitors each month, after the campaign has ended, you will still be getting 10,000 new visitors to your website, add to repeat visitors, do your own math. No other advertising campaign can claim that. 

Is your website attracting the right type of customer? E.G. a visitor that wants to buy whatever it is you are selling. It’s all very well getting tonnes of visitors to your site, but if they aren’t ready with their credit card, then they may as well not be there.
As the world wide web is essentially a very large database of websites all vying for your visitors attention, your customer is more than likely going to use a search engine to locate websites that might suit their needs, based on phrases or words they enter. As you can imagine, if your site is not optimised then it is more than likely going to be lost in the other 20,000,000 odd pages competing with yours. This is where Search Engine Optimization comes in.

Basic SEO will get your website into search engines and often that can be enough, but sometimes the words that are optimised aren’t words people are actually searching for, so although you are ranking #1 on the first page in Google, no-one actually searches for that phrase, hence your website isn’t being found. A good SEO will spend time finding out what words your potential customers are likely to use. Adequate research and wording changes can make a lot of difference to whether your website is found in search engines by your target audience and potential customers. 

The good news is that SEO doesn’t need to cost that much; in fact for a small website it can cost from as little as $500 - $1000. Ongoing costs just like the other marketing techniques you use for your business, there are, or should be, ongoing costs. A good SEO will also offer to do ongoing work on your website, monitoring how your site is ranking in search engine results, and making improvements if they feel it could do better or to stay ahead of the competition, again, it’s often costs less than you would think.