CSS cuts down a lot on the amount of code on each page and therefore, makes it easier to maintain, have more accessibility and make it more search engine friendly. There are things that can be done with CSS which cannot be done using just HTML, such as having all the code for styles in one external sheet, making editing easy.
Every modern browser will soon demand it. Web pages are being served on palm tops and cell phones, not just your standard browser anymore and who knows what is on the horizon. CSS is being used as the norm for these new versions of display in every horizon.
Web pages must be compatible enough to change with the formats and CSS is one way to do that. We cannot know all the changes that will take place on the information highway as it is forever changing, but CSS style pages are the wave of that future and therefore as web designers, we have to stay on top of those changes if we want our pages to remain viewable.
Cascading Style Sheets are being forced on the web community by the so called “standard” people, because it is supposed to be 25% to 50% faster, easier to serve up and so they say, easier to edit.
Although considering that CSS format is not a human format, with all its odd indents and spacing, it is harder to read and make changes to. It is granted, changing the directions at the top of the page which affects all the classes is easier, but not to make word and image changes. The layout can be complex and hard to follow for a human, even if it is supposed to be easier for a machine.