I've done PE-training both with and without warming up for a 3-month period at a time to see what the difference is. I found for myself, when warming up properly, which only takes 5-10 minutes. I get a better stretch and a better result(gain) in a shorter amount of time. IMO, warming-up is crucial to any physical endeavor whether its PE, sports, singing, dancing, and so...
...it's loosens the issues to have more give and less resistance to when excessive stress/pressure applied. In turn, a more consistent and better performance each time you PE-train.