As for me, the spy makes sense only at the very beginning, when you just don't have the units to secure all the lanes and don't have the spells to act interactively. In the later stages lanes are balanced. And the boss fight always has the King on the central lane.
The joke was about quantum mechanics XD
Basically in quantum mechanics there is a 'wave function' where a superposition of two or more states could be. When you observe that state, the wave collapses and the identity of that superposition is defined.
We'll use Schrodinger's Cat for the example.
Imagine, if you will, a box. The box is sealed, and it is impossible to see inside without opening the lid. Within his box is a cat, and a radioactive molecule. A radiation measurement tool is pointed at the molecule, and connected to a hammer suspended over a glass vial of deadly cyanide gas. When the radioactive molecule decays, the sensor detects it and drops the hammer, smashing the vial, releasing the cyanide, and killing the cat.
The cat has no way of interacting with the system, and is completely at the mercy of that random process of radioactive decay.
Until you open the box, it is impossible for you to know if the atom has decayed or not, and therefore if the cat is alive or not. This is called a superposition.
The atom is in a superposition of decayed and not, the vial is in a superposition of smashed and not, and the cat is in a superposition of alive and not.
The cat is now quantum-ly entangled to the radioactive molecule.
Until you open the box, it is impossible to know if the cat is alive or not. That is the 'wave function.'
When you open the box and observe the situation, the wave function / superposition collapses. At that moment, the cat is either alive or dead, rather than a superposition of both.
Tying back into the game, until you spy on the enemy, it is impossible to know where the enemies are. So, if it changed based on where you put your troops, you wouldn't know because you never observed the state of the enemy. Once you observe it, the state is defined and can't change. So if you observe the enemy before placing your troops, the enemy can't change to try to beat you, allowing you to stay one step ahead of it.
TLDR: I'm a nerd, it's funny, laugh or else.