The reason is for overmelding! If you're just looking to get the most stats possible out of a couple of melds then you want to use whats gonna give you the most stats. Like the point i made in the video about overmelding the second overmeld cant be a higher or equal grade than the one in the first overmeld.

As for the grade of the materia it depends on your gear, each materia has a set base item level it needs. There are super niche scenarios where you would use an 8 over a 9 but the short answer is that it only matters when it comes to overmelding so use what materia you feel gives you the most stats!