The standards of the W3C are not really standards, but merely recommendations that have become widely accepted as standards. It brings standards with it that will allow your website to be structured and read on most any browser safely and give you some direction for design.
Guidelines for a great markup include DOCTYPE, headers, titles, specifying the language that you will use to run your website, as well as some other features of the webpage. Generally, it is good to have less in your markups and keep it simple.
The benefits of standards are that as a designer we have something to go off of. We aren’t just fed a project and then let out in the wild to fend for ourselves. It gives us a starting point that we can grasp onto for the beginning of our work and then it allows us to mold the standards to our liking as we see fit. W3C’s standards are just good guidelines so we have some direction of how to begin at the start of our work.