This allows developers to write code completely outside WordPress, and then have it run at certain times and places inside WordPress. You can dramatically change the functionality of the platform as well as edit the output from nearly any part of the site.
(Google)
Another reason is that is open you can use:
- Bootstrap or any othe CSS framework
- cooperate with whatever you need
in order to create a theme or an app (plugin)
You can speak with database with
- wp_query and the family commands
- directly with other DB access commands
You can create forms to communicate with DB or
- with form plugins or other Gui generators
- or with pure PHP
and many more because it is open
