It's difficult at first to imagine why website development and SEO are so closely linked, and this is more so during the initial stages. Designing the look and layout of a template seems completely separate from the technical SEO elements, but joining the two together from the start is important. When laying out key parts of the graphical design it's vital that SEO has an impact.
For example, every page has some sort of logo and more often than not a header too. This header should be carefully allocated alt tags to the images, and the main header menu section needs thought for the phrasing of the sub sections. These words will appear on every single page on the site so it's clearly going to effect keyword density in some way.
The size of images is another consideration. Having too many pictures in the design and branding is a mistake, especially if they are large (file size). Also when a website is new to the search engine robots, too many links on a page will be an issue for them. This is true of both internal and external links, so SEO plays a part in this element of design and laying out the menu.
Most business owners will consider some sort of SEO in the future, and for many reasons it's always better to get that evaluation and thought process going as early as possible. This avoids the possible cost and inconvenience of changing the core of the website design at a later stage.