You could say that about any collectible. In the end, things are worth whatever people are willing to pay for them, and it happens that sealed copies are rare and therefore collectors prefer them over a loose copy. It's useful to have a grading system so you know what the condition of something is without seeing it in person, as long as the people doing the grading are honest. You certainly can't trust online sellers, so a grading system is the best way to know.
Of course, Wata Games is not exactly a shining example of honesty, but that's besides the point.
Personally, I don't find any additional value in a sealed copy, but I'm not a collector. I understand why collectors might, especially if it happens to be a game or a console they have good memories with. Probably wouldn't be worth much if it was shovelware garbage, so being sealed or rare certainly doesn't automatically mean that it's valuable. Only if it's desirable to begin with, being a sealed copy can multiply how desirable it is to a certain audience.
Complete in box I get, there is a lot of nostalgia to having those old booklets and adverts that we all used to read when we couldn't or weren't allowed to play the new game we just got, and I have good memories from that.