The HTML Macro is disabled after a clean install, due to security reasons, refer to https://confluence.atlassian.com/display/DOC/HTML+Macro
The HTML Macro is very flexible for entering style directly at a page, typically because it is the only place it will be used, otherwise create a Macro
This is a sample hiding the Menu bar:
<style> .ajs-menu-bar { display:none; } </style>
The HTML Macro can also contain JavaScript:
<script> alert('Welcome to my Page'); </script>
or to create objects around alot of other (Confluence Objects) - this sample creates a DIV around an Image, and another macro can then manipulate the DIV: