I think any time you get a (supposed) non-bias opinion working between two parties, it can be extremely useful.
(Keep in mind this copy of Bond is not a good example of this, because it's so obviously wrong.)
If I own something, I may give it higher marks because I have skin in it. Someone who's buying might go lower, to devalue my item. However, the third party should be able to provide a more accurate evaluation because they're unbiased.
It's like having your home assessed, or going blue-book value... It is supposed to take some of the guesswork out of valuation, and can also be used to validate one person's assessment of their own item.
All of this is only necessary if you care what other people think about your high condition stuff or if you're a buyer/seller. If you have your own opinions of your collection and don't care what an observer thinks, then grading is 100% stupid af.