We have Ideal gas constant for ideal gases. But ideal gas is very hard to make or to be found. So we have real gases in nature. Now there is 'real gas equation' (also called 'Van Der Waal's equation') for real gases which is given by:
$$(p + a/V^2)(V-b) = nRT$$
Here a and b are constant parameters that are determined empirically for each gas. Also 'a' tells you about attraction of molecules and 'b' tells you about volume covered by the molecules of gas.
Now couldn't we just change the value of gas constant to meet the ideal gas equation. Why there is need for two constants if we could just change the value of the constant which was already there.