To begin with, we are all different. For example, the “correct” proportion for the male shoulders is for it to be three heads wide, but I have seen a photo of an athlete who had broad shoulders and a small head, and his shoulders were five heads wide.
Add to that, you can be expressive and say something about the thing you are drawing by controlling the proportions, exaggerating certain things, and downplaying other things.
When you let go of “correct” proportions, drawing becomes easier, as it doesn’t matter if a line is off by a bit. It does matter when you want to be accurate: when a line is slightly off, it jumps out at you. When it is wildly off, it looks like it was meant that way!
And freehand drawing is just fun. For me, measuring is less fun than freehand drawing, as you see me do in this video.
In the video, I used a fashion photo and drew the model in three different ways, where the parts had wildly different proportions in each drawing.
Radically changing proportions can make the drawing more interesting. Proportions can be an artistic choice. You don’t have to copy a reference image.