As a simple example, it's very easy, when starting a company, to issue personalized email addresses to early employees and then people communicate using those email addresses. It's perfectly fine to email the CTO at [email protected], because everyone knows everyone else and it works.

As you grow large, it becomes important for people to address roles rather than individuals. This way, if people leave their role, they can (semi-transparently) be replaced by someone else taking that role who will then continue to receive all of the same emails, be able to respond to them, etc. So then it becomes important to have e.g. a [email protected] address. When the CTO takes a vacation, their email gets routed to someone taking over their duties, you don't need to communicate to everyone to start emailing [email protected] instead.