We have to begin somewhere in our description of matter and how it behaves. One principle is start as simply as possible. A second principle is to develop theories that can be expanded as we obtain more data. The ideal gas law is useful because every gas can be considered ideal in the limit of low pressure. This is because in that limit the molecules are far apart and their mutual interactions are weak simply because the distance is so large. This is a nice feature in the theory when every material can be made to obey it in some limit. The ideal gas law (PV = nRT) relates three variables of state, which are P, V and T. The number of moles, n is not a variable of state.
