I started putting a line through my 7’s and Z’s a few years ago in my senior year of high school. I just think the line looks cool, that was my only reason.
I do put a line through my 7’s. I remember starting sometime in elementary school when I saw one of my friends doing it, and I thought it was cool, so I started doing it as well, and it’s just become habit since then.
As for the line through the z, sometimes I will do that, but most of the time I don’t.
I don’t, however, I have a friend that went to Preschool and/or Kindergarten in the UK, where she learned to put lines through them, and then got yelled at for writing them “incorrectly” when she moved to the US.