This question is at the core of any website I build or upgrade. The follow up question is, "What do your users need, and how can your site meet that need?"
User interviews, as well as writing user stories is a key part of building your website. Put yourself in your site visitor's place.
In the 1980s there was a movement in publishing technical documents to separate the information in a document from it's layout and style. This movement led to the adoption of SGML (Structured Generalized Markup Language). Later a refinement that allowed linking documents was introduced. This HTML (HyperText Markup Language) spawned the birth of the web.
Sounds good, but what happened next? Well the early browsers did not do much to support defining the layout and style of documents. Also HTML mixed in presentation tags such as <span> and the dreaded <blink> tags. To get multi-column layouts, headers and all the other stuff designers liked, well we cheated. We took the <table> tag which allowed you to array data in a table, and stuffed paragraphs of content, images, and even put tables within tables within tables.While it looked sweet in browsers of the day, it made life difficult. Text readers for the blind had a hard time figuring what to read, order would get mixed up. Since most users over 50 have some vision issues, being able to change text size is also key.
Over the last decade we have moved back to the idea of a content only document (XHTML) which we supplement with separate style sheets (CSS) to describe how the page looks, and where stuff goes. With a simple switch of styles we can have a mobile template optimized for the iPhone or Android. Pages are optimized for text readers, and work at many text sizes.
So if you want to reach the broadest range of customers, Cascading Style Sheets can make it happen.