To answer the 1st part of the question I dug my "Inside Microsft Dynamics AX 4.0" book off my shelf and it had the following to say on why you would use the global class.
The compiler treats methods in the global class in a special way. It allows you to omit the reference to the class name. For example these two lines have the same meaning
Gobal::info(...);
info(...);
This compiler feature helps make you X++ code more readable. If you find yourself referring to the same method from many places or repeatedly implementing the same trivial functionaility, you should consider adding new methods to the Global class.
It is a great book you might want to invest in... You can find it here on Amazon