Why use It.is or It.IsAny if I could just define a variable?

Using It.IsAny<>, It.Is<>, or a variable all serve different purposes. They provide increasingly specific ways to match a parameter when setting up or verifying a method.

It.IsAny

The method set up with It.IsAny<> will match any parameter you give to the method. So, in your example, the following invocations would all return the same thing (ReturnSomething):

role.GetSomething(Guid.NewGuid(), Guid.NewGuid(), Guid.NewGuid());

Guid sameGuid = Guid.NewGuid();
role.GetSomething(sameGuid, sameGuid, sameGuid);

role.GetSomething(Guid.Empty, Guid.NewGuid(), sameGuid);

It doesn’t matter the actual value of the Guid that was passed.

It.Is

The It.Is<> construct is useful for setup or verification of a method, letting you specify a function that will match the argument. For instance:

Guid expectedGuid = ...
mockIRole.Setup(r => r.GetSomething(
                 It.Is<Guid>(g => g.ToString().StartsWith("4")), 
                 It.Is<Guid>(g => g != Guid.Empty), 
                 It.Is<Guid>(g => g == expectedGuid)))
         .Returns(ReturnSomething);

This allows you to restrict the value more than just any value, but permits you to be lenient in what you accept.

Defining a Variable

When you set up (or verify) a method parameter with a variable, you’re saying you want exactly that value. A method called with another value will never match your setup/verify.

Guid expectedGuids = new [] { Guid.NewGuid(), Guid.NewGuid(), Guid.NewGuid() };
mockIRole.Setup(r => r.GetSomething(expectedGuids[0], expectedGuids[1], expectedGuids[2]))
         .Returns(ReturnSomething);

Now there’s exactly one case where GetSomething will return ReturnSomething: when all Guids match the expected values that you set it up with.