Markup could be understood correctly as a set of tools for formating portions of content. Then styled up accordingly later on using a completely separate tool, namely CSS. (Cascading Style Sheets). A great method of separating page content and style. Just to note: Markup certainly was not set out as a way of styling web documents, a commonly misunderstood myth. Markup is the bed bone of any web page so getting it right first time and giving your content meaning, makes updating/improving vastly easier at a later date.

Making your documents more semantically, deems other benefits like being smaller in file size, therefore reducing bandwidth costs on the server and with style-sheets being cached reduces return trips therefore load times are considerably quicker. They also raise higher importance in search engine results.

This just touches on surface of why its important to produce semantically valid markup, however, provides enough insight as to the reasons why everyone should be making it a number 1 rule when working for the web.