Scores are something objective whereas verbage on the lines of "it's a good game" is subjective. Anyone would understand that a score of 8/10 or 9/10 means it's a great game no matter what the frame of reference whereas a "good game" could be 6/10 or maybe 8/10. Scores help in making the reviews, which are opinions anyway be it movies or games, objective and a score does help folks get an idea whether the game is worth buying or not at a glance.
If I've spent a lot of time weighing up the relative pros and cons, and written a detailed explaination of what I thought, the last thing I really want is people deciding to buy the game "at a glance" based on a score, which is open to their own interpretation, and then blaming me if they don't like it. A score gives it a degree of officialness that opinions have no place carrying.
It does, however, allow people in the comments to argue that although it was a 7, it read more like an 8. Or suggest that the publisher paid for the 9, when the game was clearly an 8. Which makes those people's lives worth living I guess.
My favourite review of all time about one of my games was a UK magazine who scored each different aspect with numbers, including the brilliant:
"Graphics: Pushes the DS to its absolute technical limits - 7/10".
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Scores are something objective whereas verbage on the lines of "it's a good game" is subjective.
I don't agree at all with this: subjective means influenced by your own personal opinion, objective means something unbiased and fair-minded. If a journo's personal taste influences the write-up it will affect the number you stick at the bottom in exactly the same ways, but without the context.
Example:
I hate eggs and review an omelette. In the review I explain that I hate eggs, so if you, like me, hate eggs, you probably won't like it either.
I review an omelette, and give it 2/10.
So long as you understand the context, a subjective opinion becomes objective.