Back in more primitive times, notebooks were generally only available with either lined or plain paper. Times gradually moved on, and squared paper became more common, escaping being typecast as 'graph paper', thought of as only being good for lab notebooks for scientists and students. Oh, we thought we were so advanced with our square grids, but then a calculator with an LED screen was once the epitome of pocket technology, and we thought our Parka coats with furry-edged hoods were so great.
But times continued to move on. Now, we have iPhones, The North Face jackets with Gore-Tex fabric, and dot grid paper in our notebooks. It takes the best features of squared grids, but makes them less obtrusive.
Because there are only dots there, instead of complete lines, they don't get in the way as much - less visual clutter to distract from your notes.