Without the explicit declaration of the class as static
it is a normal class and can be instantiated unless you prevent it ( see how in that question ). So it has different implications to have the static
modifier in addition to the documentation that can only have static members.
But one must ask why creating a normal class when all its members are static. Is there any reason? If you do not find it, you are doing something "wrong."
Remember that all members need to be static, not just methods.
In C # 6 you can import static classes, not normal classes, with:
using static System.Console;
using static System.Math;
lets you use:
WriteLine(Sin(12));
If these classes were not static, you could not do this. Just to cite an example.