Question: what is the function of pretrained weight

Hi, I am trying to train a custom yolor model but have some question towards the command line parameter.
It seems we need to add a pretrained weight( commonly use yolor_p6.pt) when running the custom model. What's the function of adding it? Since the classes that yolor_p6.pt used are different from mine. It still works when I left the weight parameter empty. What does it imply? 
Thank you. 