We have one on the Yanmar between the tank and the RACOR just to use for filling the filter and if, necessary, purging the air out of the system.

We leave it OFF except when doing either of the above.

If you have an electric fuel pump, the advantage is (as Todd says above) that you can run fuel through the system without leaving any air in injectors as long as you don't try to "crank" it. The fuel pump, in my experience will tend to simply cycle it through the ruturn and not introduce those pesky bubbles.