Too often, they don't. Many web site designers will make you a site and claim to address search engines and get you traffic. In many cases, that does not really happen, at least not up to your expectations.
What are your expectations. If they are realistic and your desired "target" is well thought out, researched and sound, you can and will get good traffic to your site. But it is not a given. It is not automatic.
j.t.a.c., Inc. has a great deal of experience in this area. I understand the reality of search engines, what phrases you might expect to get traffic from and how to build your site with those capabilities built into the site.
For details about how to get your site into position on search engines, click on the heading "Suggested Preparation", or click here.
It is basically hard, no-fun, no-glammor work which achieves this important goal. Proper analysis of what phrases are used by your desired viewers and of those, which phrases you can realistically expect to show up on a search result for. This process should proceed any site production. It is the first step, a very important step.
For details about how to get your site listed on search engines, click on the heading "Search Engine Placement", or click here.
This placement and/or submission of your website is not all that difficult. All the work is done before submitting or buying your way into search engines. There are of course details. Read this section for a brief understanding of current Search Engine Placement options and methods.