Why would we need to know $c_p - c_v$ I understand that it's easier to calculate $c_p$ experimentally since we pretty much experience  constant pressure conditions all our lives. It's also more difficult to calculate $c_p$ mathematically since $$c_p = \Bigg{(}\dfrac{\partial H}{\partial T}\Bigg{)}_p$$ Mathematically $$c_v= \Bigg{(}\dfrac{\partial E}{\partial T}\Bigg{)}_V$$Is easier to calculate, But I've just proved $c_p - c_v$ and it took me a while so I was wondering why would we need to know $$c_p-c_v = \Bigg{[}p + \Bigg{(}\dfrac{\partial E}{\partial V}\Bigg{)}_T\Bigg{]}\Bigg{(}\dfrac{\partial V}{\partial T}\Bigg{)}_p$$
Basically why would we need to know $c_p - c_v$ at all. 
 A: The difference between the heat capacities is the definition of the specific gas constant:
$$ R_{\text{specific}} = c_p - c_v $$
And so depending on your use-case, that could be handy. Usually we use it the other way around to get either $c_p$ or $c_v$ from the universal gas constant and molecular weight and whichever heat capacity we know.
A: The importance of the quantity $c_p-c_v$, where $c_p$ is the specific heat for a constant pressure and $c_v$ is the specific heat for a constant volume, is given by the Mayer Relation that relates the specific gas constant to the specific heats for a calorically perfect gas and a thermally perfect gas.
Generalizations
For more general homogeneous substances, not just ideal gases, the difference takes the form,
$$ C_{P}-C_{V}=VT\frac{\alpha^{2}}{\beta_{T}},$$
where $C_{P}$ is the heat capacity of a body at constant pressure, $C_{V}$ is the heat capacity at constant volume, $V$ is the volume, $T$ is the temperature, $\alpha _{T}$  is the thermal expansion coefficient and $\beta$  is the isothermal compressibility.
From this relation, several inferences can be made:


*

*Since isothermal compressibility $\beta _{T}$ is positive for all phases and the square of thermal expansion coefficient $\alpha$ is a positive quantity or zero, the specific heat at constant-pressure is always greater than or equal to specific heat at constant-volume.
$$C_{{P,m}} \geq C_{V,m}$$

*As the absolute temperature of the system approaches zero, the difference between $C_{P,m}$ and $C_{V,m}$ also approaches zero.

*For incompressible substances, $C_{P,m}$ and $C_{V,m}$ are identical. Also for substances that are nearly incompressible, such as solids and liquids, the difference between the two specific heats is negligible.
