Whenever you want to annoy users.
More seriously, since this opens a new window/tab, it should be used sparingly, in my opinion.
Most modern browsers have some option (ie: middle-click on hyperlink) to do this for you, so I personally prefer allowing the user to handle this themselves.
However, if you are working on something where there is a non-technical reason for this, such as a sales-oriented site, it’s often desirable to open a product brochure in a new window, or something along those lines. However, just be aware that overdoing this leads to my pseudo-joke response above….