Adapted from On Web Standards

Relative Sizing is a Web Standard: in fact, using relative units in general (for sizing, positioning, font size, etc.) are a basic part of Web Standards. This makes it easy for multiple types of devices to render the content in a way that's appropriate to the user agent (browser, cell phone, PDA, etc.) and to the user (whether perfect vision, or impaired)

Bandwidth savings: The front-end code is simpler and faster to download since it doesn't have all that extra table, font, and presentatioal markup. This reduces the load on servers and networks, and therefore reduces costs. A single stylesheet downloads once into the user's browser and is reused for every screen thereafter.
Cheaper maintenance and easier redesigns: When it's time to redesign a portal, it will be approximately a million times easier to edit a stylesheet than to change tables on every single page (or even on a whole bunch of templates).
Future flexibility: We don't really know what protocols and devices are coming down the pike (video iPods, other mobile devices, ENUM protocol, etc.), but if your content and functionality is separated from presentation, it won't be hard to port content or functionality over to any kind of device or technology.
Improve the User Experience: Faster Web pages will make our applications more responsive and will increase customer satisfaction (standards-based pages are both lighter (less code) and render faster).
Better search engine rankings: There is some search engine advantage to having your content be simple and pure and not mucked up by lots of other markup.

It's easier to change requirements mid-stream: Projects have a tendency to change. Standards make change less arduous. Don't make it any harder on yourself than you have to.

User agent interoperability: XHTML is geared towards all sorts of different media devices (mobile phones, PDA’s, etc.). Stylesheets can also be created for different media types (for printing, for example).

Degrades "gracefully": Basically this means that when something breaks in Netscape 2.0, it might look uglier than homemade soap but at least you'll be able to access links, and read text in the order that it's meant to be read.

Goodbye nav bar GIFs: In the old days, it was common to make your navigational menus as GIFs, which meant that every time you wanted to change your navigation bar names, you had to recreate an image, slice and dice it, rebuild the HTML, edit templates etc. This is still common, and it's still a pain. CSS lets you make good-lookin' navigation bars that are HTML text, which reduces five steps to one.

Divide work by expertise: because the presentation and logic layers are typically maintained by separate teams, dividing the code into logic and presentation is congruent with most company's team structures.

Universal accessibility: We have both a moral imperative and a business imperative to provide functionality to a wide variety of users who access our sites via a wide variety of user agents. Standards ensure universal accessibility.