The purpose of warm up is to infuse blood and oxygen into the areas you’re about to stretch. It’s just like any exercise, whether you’re running, lifting weights, or whatever. If you go straight into it without warming up you risk injury, and it won’t be as effective.
Keeping your dick warm as you are stretching, jelqing, pumping or squeezing also seems to yield good results, but that’s the subject of other threads.
If you don’t have a way to warm your parts through a rice sock, infrared lamp, warm washcloth, or again, whatever (I use one of these filled with warm water) you can get some warm up by just pulling and shaking your package. For example: cup your balls in your hand with your dick between your thumb and index finger and jiggle it all around. Or grab the end of your dick and and pull out has you take your other hand and massage your ligs.