When does the _self frame target becomes useful or worth using it?
Isn't it (always?) the default behavior?
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.
_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>...
<base target="_self"/>
Was very handy for me when I had a page I wanted to load into Firefox's sidebar. I wanted any links clicked to stay in the sidebar instead of opening the links in the main window. Adding the above to the <head> fixed me right up and all the page's links stay within the sidebar.
来源：https://stackoverflow.com/questions/15771047/when-to-use-target-self