Search Engine Optimization
Whether designing for a PDA or a visually-impaired individual, the concept of Universal Access (which includes numerous modalities such as phones, print, web site all coming from the same document as well as access for those with specific disabilities) is far more easily achieved using the techniques advocated by Web standards, making a document significantly more style-able and scriptable for those multiple reasons.
As sites grow, and they have a tendency to do so, a great deal of effort can be saved by having an existing standards workflow and corporate/organizational design guide. This empowers people to scale, refresh and even redesign sites with greater ease than in non-standard design, in which each page "carries" its design with it rather than being controlled by external style sheets. By separating document, presentation and interaction behavior on the technical side, a very significant amount of time, money and frustration can be avoided as a site grows.
Improved User Experience
The lighter-weight files associated with standards-based design mean an improved experience for the user. In order to be effective, sites have to be easy to use and allow the user to get to their goal quickly. A clean infrastructure and the smaller files typically resulting from a Web standards workflow can have a significant impact on the load time and responsiveness of a site, keeping your users happy customers.