It amazes me what people will ask on the internet:
Someone’s helpful answer: It’s not.
I live in Europe (Netherlands), and we have both. There are notebooks with vertical lines, and there are notebooks with grids.
For normal day-to-day usage we a simple vertical lines notebook, but for some occasions the grid is simply handier, like when there are lots of graphs to draw. So, we also use each notebook when applicable, and the vertical lines one is used in most occasions, just like in the US.
I’m glad to learn this interesting information about the strange notebook-use habits of that exotic species known as Europeans!