The w3c standards help designers and developers make there sites as viewed and functional as possible. They provide knowledge for web design, web devices, architecture, tools and much more.
Markup is how the content of the page is described. They can be standard generalized markup language, SGML or extensible markup language, XML. These are how the document is marked up. They are both very similar, however XML is slightly more precise than the other.
Using the standards will make smaller file sizes, increase portability, better accessibility and precise control. having smaller file sizes will increase download time and you can reach people quicker. With an increased portability one can easily scale from a large monitor to a small phone screen. giving you more control allows for you to change things such as line-height or padding properties that can be very helpful.