Q:

When to use target="_self"

When does the _self frame target becomes useful or worth using it?
Isn't it (always?) the default behavior?

A:

The default can be changed by using the <base> tag in the <head>:
<base href="http://www.mysite.com/" target="_blank"/>

In this case, you can use target="_self" on a link to override the target set by base.

A:

_self target value could also be useful in conjunction with AngularJS's HTML5 location mode when you do need to force full page reload for links with the same base as your single page app.
https://docs.angularjs.org/guide/$location#html-link-rewriting

In cases like the following, links are not rewritten; instead, the
  browser will perform a full page reload to the original link.
Links that contain target element Example: <a href="/ext/link?a=b"
  target="_self">link</a>
...

