In my opinion, PE is like training other muscles. When people go to the gym they usually follow the order of a quick warm up(warm wrap), stretch (manual penis stretching), lift weights (jelq), warm down(warm wrap) & finish with stretching(penis stretching again).
I don’t know whether the stretching of the penis helps but in my mind perhaps it relaxes or expands the corpus so that it can take more blood flow to have a better jelq or perhaps it’s just to get the penis ready for the jelqing. All I would say is be careful if you jelq straight away, make sure you don’t go too intense.