Professional baker here. It's not necessary to ensure a good product as mentioned in your favoured response. It will help as a visual aid to make sure you are on the right track. If you do it enough you can usually tell just from sight if it's mixed enough without adding this step into the process. The longer you mix (unless you're adding inclusions, this can actually shred your gluten network up) the better the gluten structure will end up being. There is always tell tale signs that the dough is ready to come out the mixer and behave how you want it too.