For higher-end cards, especially Holos, it's not a bad idea to get subgrades. It helps the owner and potentially the buyer know why the card got the grade that it did.
For example, I recently submitted a Shining Charizard card. It was in a binder all this time and it came back an 8.5. I was curious as to why so I looked at the subgrades, 3 of them were 9 but corners was an 8 which dropped the score down. And looking back when I was submitting I did remember noticing some whitening on the back of the corners and some weren't as round as pristine corners should be.
So that's pretty much it, less guess work needed to understand why your card got the grade it did.