Why should follow web standard ?

We know that W3C School is an organizations which creat Web standards and guidelines.
However, why we need to follow those standard and guidelines?
Why do not just create our website by our wonderful and creative imagination?
We should make clear the this concept before we become a professional web designer.

• #1 Accessibility
  
  There are lots of web browsers nowadays. It is important for your visitor to visit your site will see a similiar webpage even if they are using different browsers. Build the website with the web standard can provide this advantage.
  
  
• #2 Futureproof
  
  We cannot guarantee the websites can be readable in new browsers or the website still workable in 10 years or even more long time. The world is changing everyday, therefore if the design is separeate the content and use the current web standard which means to build the content and information in one file and contain the design, structure of the content displayed in one file.
  
  We can change the design with the design file without change all the websites. It save money and time in the business world.

There are a lot of reason we need to use the web standard to build the website. Nevertheless,
many people will misunderstand the use of web standard.
Someone think it will increase development costs, however, it does not take much time for the web designer to follow the standard and it is easier for the new developer to join in the project in every phases.

Reference:

Ten reasons to learn and use web standards

Why Web Standards are Important – even to Small Business Websites

10 mistruths about Web Standards