Hello guys,
I am a sports trainer and physiotherapist from India. Pre-exercise stretching is an important part exercise routine. It has to be included as a part warm up and cool down/warm down. Dynamic stretching can be used as a part of warm up schedule and static stretch can be for cooling down.
For warm up it helps in lengthening so that during exercise it avoid injuries to the muscle and stretches included in cooling down will avoid soreness in muscles and wash out lactic acid. I hope that i made some sense.

Cheers.