At very high pressures, the ideal gas law can no longer descibe accurately the state of those "real gases." Because, in a real gas, the volume of the gas particles at high pressures needs to be taken into account. In reality, those particles are not independent of each other i.e. they really do effect each other (attractive forces). Therefore, in order to account for the change of volume observed at high pressures, one will need to utilize the van der Waals Equation.