Note that this is not simply "built in to most browsers" anymore. It is required in order to be HTML5-compliant.
about:blank is used in many places in the HTML5 standard, for example in § 4.7.2:
If the value of the
src attribute is missing, or its value is the empty string, let url be the string "
If you search the living standard, you'll see that it is generally used as either a fallback URL when there isn't a more appropriate one available, or as the default URL in an initial browsing context.
In layman's terms: it's like the blank document in Microsoft Word, or the empty canvas when you open Paint. It exists to provide a starting point when you aren't opening anything more specific. Of course, most of the time a more specific homepage replaces
about:blank these days.