In general usages of a macro produces an element, and every element has an ID.
IDs must be unique, and they are used as the target of internal cross references.
E.g. due to Section "Automatic ID from title", the elements:
= Animal

== Big dog

I like <big dogs>.
would have IDs respectively:
  • animal
  • big-dog
Such IDs are almost always rendered as HTML IDs as something like:
<h1 id="animal">
<h2 id="big-dog">
and can therefore be linked to in a page with the corresponding fragment: