It is "de rigeuer" here to move the key.snk file generated when a new project is created from its default location at the bottom of the Solution up into the Properties folder.
This is not just a local "superstition"; it seems to be the "accepted method", as seen here
But why? That book does not seem to explain the raison d'etre for this action...
It makes no difference where the key file is located as long as the assembly is signed.
The author explains why she/he does that:
"As projects become more complex, a lack of vertical space in the Solution Explorer makes projects more difficult to work with"
So the reason seems to be to hide the key file inside the properties folder to save some space on the visual studio Solution Explorer display.
My personal opinion is to ignore that as that will confuse another developer which is expecting to see it in the default location: in the project root.