_ is a special identifier you can assign anything to but never read from. In the first example you gave:

var _ I = T{}

There is no way to access this variable so it will be optimised out of the resulting program. However, it could cause a compile error if the type T is not assignable to the interface I. So in this case it is being used as a static assertion about a type.

The second case is more common. While it might seem strange to throw away the result of a function call, it can make more sense in functions with multiple returns. Consider a function foo that returns two values but you're only interested in the first? You can use _ to ignore the second:

a, _ = foo()

You could get the same effect by creating another variable to hold the unwanted return value, but this feature means you don't need to worry about picking a unique name for it.