For readability of course. As a developer you probably spend more time maintaining an application than you do writing a new application. What this means is that your time is spent looking at code.
More than likely, it was written by somebody else. If not then it was written by yourself. Either way comprehensive need to be at it most productive. If the signal-noise/noise ratio is very low then your comprehension is going to be low. We need to understand content. Any presentation mixed in with the content means a lot of noise.
CSS is a way to reduce the noise.
Let’s look up a contrived example of creating a navigation panel using static HTML: