Couldn't we simply skip this step, so that any IP inside our internal
network gets translated?
No, not all addresses on a network need to be translated. Businesses often have public addresses too, and translating those would be unnecessary and counterproductive. There may also be networks inside a business that should not have the possibility of Internet access, and not allowing translation of those addresses is also a security step.
Do not look at this from a home user perspective, but that of a large business network.