Wildcarding accountId in serverless.yaml

The serverless.yml I currently use has several iamRoleStatments that reference resources where * is used for accountId. There are several posts here explaining how to handle referencing the real account id. The question is why should I care. What happens if never give real accountId but leave *. Are there any risk? Why would someone specify an exact number here

Is my understanding correct that accountId is one for the whole organization and is not operating user specific?