This node converts a direction to a rotation, A vector that defines a direction is inputed and a corresponding rotation is output.
To better understand how this node works, You can think of it as aligning the local Track Axis of the object to the input Direction Vector.
It is true that the selected local Track Axis is locked to the input direction vector, but that means the object can still rotate around that vector while maintaining its direction. This node enables you to guide the object rotation around that axis using another vector, Guide Vector and Guide Axis.
You can also think of the guiding feature as aligning the local Guide Axis of the object to the input Guide Vector.
Here is an example where the Track Axis is set to Z and the Guide Axis is set to X.