In the previous three examples of theme hooks, we encountered the concept of attributes in the context of using them to render HTML elements. Attributes here are understood in the same way as with HTML. For example, class, id, style, and href are all HTML element attributes. Why is this important?
The reusability of theme hooks makes it so that we cannot hardcode all our HTML attributes in the Twig template files. We can have some, including classes, but there will always be the case when the business logic will need to inform the theme hook of certain classes or other attribute values to print on the HTML element, for example, an active class on a link. This is why, we have this concept of attributes.
Most theme hooks you'll see will have attributes in some form or another, usually the variable being called $attributes, $wrapper_attributes, or something of this...
