Bandwidth & rendering
Table less design means less bandwidth. If you link a CSS file, a browser will cache it on 1st load of a page and on further navigation only page structure itself is being downloaded. In addition to traffic cuts it significantly reduces page latency as well.
Accessibility
Prioritizing Cascading Style Sheets will also ease the process of porting a site to various platforms, e.g. mobile phones, handhelds, etc. or usage, e.g. printing. Not to mention making the site reachable for a screen reader or braille device.
Maintenance
With CSS site layout is kept apart from its structure and is centralized. That makes site design a lot easier to change. For example changing 1 single digit in CSS may increase or decrease font size of an headline on each and every page of a site. CSS is a reusable resource as opposed to the inline definitions of HTML. This is especially useful as you will be able to customize your website very easily.