The ratio laws compare a gas to ITSELF, before and after. The ideal gas law needs no before: $PV = nRT$ — P V equals n R T — pins down a single state, because it knows something the ratios never asked: how MUCH gas is there. One state, four variables, and the constant $R$ refereeing between them.

$R$ is one constant with several suits of clothes, and choosing the suit is the units test as a question. With pressure in kPa and volume in litres, $R = 8.314\ \mathrm{kPa \cdot L/(mol \cdot K)}$ — the suit this course wears. With atmospheres you would reach for 0.0821 instead. The number is not the law; the AGREEMENT is. Feed kPa to the atm suit and every answer comes out 101 times too shy — dimensional honesty is a one-way check, and it catches this one instantly.