Question

Why is G added to the equation F=Mm/r*r ?

Answer 1

because it is the gravitational force

Answer 2

Gravitational force is PROPORTIONAL to the product of the masses and inversely PROPORTIONAL to the square of the distance between them. To remove the proportionality and make it an equality, there exists some proportionality constant which is multiplied. This has experimentally turned out to be  6.674×10−11 N⋅m^2/kg^2, and this is called G.