What is about:blank for?

It is simply a blank page built into some browsers. Some people just like to have a blank page and not have to see/load anything else when they start their browser or open a new page.

The about URI scheme is designated for internal browser use. It originated in Netscape Navigator, the browser that gave rise to Mozilla Firefox. This includes about:blank.

Per IETF RFC 6694, about:blank is reserved for a blank page. Essentially every browser that supports about implements about:blank and it is pretty much the same everywhere.

Because about:blank is little more than a blank internal page, it doesn't depend on any external resources and serves as a good default on metered connections or slow systems.

about:blank is the most fast and clean way to open a browser.
You do not use up your bandwidth, nor wait for a page to download, and you will (presumably [see below]) not execute any code hidden in any page.

3. "about:blank"

This document defines one well-known token: "blank". The "about:blank" URI refers to a resource represented in the browser by a blank page. ^[1]

However, even the most elegant and apparently clean pages can hide hidden code (but not necessarily malicious code): for example, go to Google and look for :-) in the code.

References

• The rfc6694 all about the about by the Internet Engineering Task Force (IETF).

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 "about:blank".

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.