Historically, why was it necessary to define the Zeroth Law of Thermodynamics after defining the other Laws? In our current understanding of science, the way we study it in schools, it seems obvious. So, what is so special about Zeroth Law? Why is it defined as a Law? What knowledge were we lacking without defining it? What was the need to define it?
If two unequal thermally insulated cylinders are connected with a pipe with a piston in it, then these two cylinders are said to be in "Mechanical Equilibrium" if the piston does not move and we conclude that the "Pressure" in both of them is same.
Similarly, if two bodies are brought in contact with each other and there is no exchange of heat between them then we say that these two bodies are in "Thermodynamic Equilibrium" and hence there must be a property which these bodies must be sharing analogous to "Pressure" by the virtue of which there is no exchange of heat in between them, and this property is defined by Zeroth Law of Thermodynamics as "Temperature". Zeroth Law of Thermodynamics defines the thermodynamic property - "Temperature".
The reason it is called Zeroth Law and not the first law because it was defined after having defined the First and Second Laws of Thermodynamics. By the time scientists felt the need for Zeroth Law of thermodynamics, the First and Second Laws had become so popular that it was not possible to change their names or numbering, so this law was defined as Zeroth Law.
It is worth noting down these different laws of thermodynamics define different thermodynamic properties.
Zeroth Law defines Temperature.
First Law defines Internal Energy and Energy Conservation.
The Second Law defines the idea of Entropy.
Third Law defines the idea of Zero Kelvin Temperature and also establishes that it cannot be achieved.