When we write a program with PHP, we can write:

• in vanilla PHP (it means using the language as it is)
• using a PHP framework (it means a set of commands that summarize the functionality) like, CodeIgniter, Laravel e.t.c.
• using a PHP CMS like WP, Joomla, Drupal e.t.c ( it means using prebuild blocks of code, or even first write the blocks)

If we write in this language, we must have libraries written to speed the procedure, but in general, it is more difficult and time-consuming.

When we use a PHP framework, it is faster and easier. The fastest and easiest framework is Codeigniter, it uses MVC (Model View Controler) model and it is Object Oriented.  If you work alone or you are two programmers go with CI.

WordPress is a framework, a blog platform, and a CMS, it is the easiest the fastest to write code and it is the most popular CMS, it has blocks of code-named plugins. Joomla has triggers and extensions, it is more difficult than WP. Drupal is the most secure and difficult.

Comparison of CodeIgniter PHP Framework with WordPress PHP Framework

Similarities of WP and CI, as frameworks are:

• both are easy
• you can build fast the code

Differences are:

• CI includes Cache functionality, WP not
• CI uses the MVC model, it is OOP

Someone can give many answers, I'll give a few reasons:

• Speed of loading the site
• Customization (to write a custom app)
• Security (you write the code, can be more secure)