Is the question which was given a comprehensive answer by Jon Davis, a former Marine Corps marksmanship instructor:
The reason is actually to help them aim faster by utilizing a technique known as “Flash Sight Picture”.
I wouldn’t advise it though. I specialized in pistols and have fired these weapons thousands of times. That said, the thought has crossed my mind. The answer didn’t come to me until another coach (from the “hood”) gave me a good reason why this technique would be used. In practice it actually does utilize one very important sighting practice, but fails overall. This is a good idea in theory, but fails miserably in the actual execution.
Full explanation with diagrams over at Quora.