To stretch the injection window to make the most of the injector capacity.

But, afaik, the injection start angle will increase to the maximum specified start angle as injection time increases, in anycase.  If you're wanting to stretch the injection window further you need to increase the maximum start angle.  Depending on the VVT/intake cam angle, you may be spraying into an only partially filled cylinder if you take that approach, which can't be that great for fuel atomisation.

To answer your question, on a stock car, there would be no reason to do that, unless the changes you're seeing are changes made when changing to a different injector.