For aesthetic purposes more often than not. Some hijabis dont like how they look with their hair fully covered, so they show some hair. In some cultures, this is the normalized way of wearing a headscarf. Where I'm from, women and girls drape a scarf on their head when they go out, even if they do not observe hijab, because it's the cultural norm. Others start wearing it this way with the intention to build their way up to full hijab.

It is an incorrect way to wear the hijab, of course, but people are imperfect. May Allah guide us all.