Why CSS ?

Making changes to the layout
CSS makes it very easy to change the style of a document. Let’s say we wanted to move the picture in the title of this page to the right by 30 pixels. This would be a nightmare in a table based design. We need to open every pages and need to alter manually, it’s very difficult.
If we have used CSS, and all we have to do is open our CSS file which stores the layout of the site, and change the number relating to the position of the image. That will change his position throughout the whole site.
The look and layout of a site can be changed beyond recognition just by altering the CSS file. This makes CSS indispensable for large web sites.

File Size
Probably the mostly useful feature of CSS is that all of the style and layout is removed from the html, so the html page size is very much smaller. The CSS file is downloaded just once by the visitor’s browser and re-used for different pages on a web site. This reduces the bandwidth requirements for your server and also makes faster browsing.

Search Engines
A search engine robot will normally consider the content in the start of your html code is more important than the text towards the end of the code. For a table based page the contents of the navigation bar will normally show up as the page description in search engine results. With a CSS page the navigation can be moved to the bottom of the source code, so the search engine displays your content instead of your navigation.

Accessibility
Separating style from content makes life very easy for visitors who prefer to view only the content of a web page, or to modify the content. These could be blind or partially sighted people who might use a screen reader to interpret a page.

Consistency
Layout and position of navigation can be completely consistent across a site. This was previously possible only using frames.