There are many good reasons to build a website but it is important that your own goals are clear. Your objectives should drive the content and the design of your site.
Many people build home pages as a hobby, purely for their own personal development. Others build them to promote business, social, cultural, humanitarian, or business objectives.
Knowledge of website development can help you in your career. Many businesses today have their own internal Intranet sites, which are simply internal Internet systems used to share corporate information. The ability to communicate electronically is a valuable skill to employers.
There is also potential for generating income from the web pages you develop. This is done by publishing interesting content that attract large numbers of visitors. You then allocate space on your web pages for third-party advertising.
Judging by the large number of websites that contain little useful content, it would appear that many web developers have not clearly defined their objectives!