From the early days, Drupal had the ability to embed PHP code in its content. This provides flexibility and functionality, most importantly, nodes and blocks can contain dynamically fetched data from the database using custom queries and displayed them in other content.
This is an easy approach to get such data without writing a module. All you need to do is assign the PHP input format filter to the node or block and paste your PHP code in it, and voila, you have dynamic content.
Code in Content: the issues
However, this approach is suboptimal and has serious drawbacks, including: