You would use multiple receive connectors to handle different classes of email in different ways.
When an SMTP session to the Exchange server is started, the most appropriate Receive connectors is chosen to handle it.This is based on the Exchange server IP address and port the fledgling session is being received on and what the IP address the calling device is using (the most specific range of IP addresses on the Receive connectors bound to the listening port wins, and selects its parent connector). Although less common if the remote computer authenticates, the authenticated user can also help elect the chosen receive connector.
Once the Exchange server has selected the most specific connector, the settings on that connector govern the following SMTP conversation. You cold for example not allow SMTP authentication (and say what authentication methods are allowed) for a particular connector, allow (or enforce) TLS (encryption), turn on/off logging of the SMTP conversation, specify what classes of devices may authenticate (users, other exchange severs etc), even customise the server name inserted into the SMTP HELO banner message .
Exchange comes with one "one size fits all" connector what sort of just works, however there are distinct advantages in further customisation. For example. For example not allowing authentication (*) on smtp connections received from the internet, logging the conversations and logging it all; or (on a receive connector targeted at selected internal IP addresses) allowing anonymous (*) email sent from your domain. Whilst still allowing the normal authenticated SMTP chatter between multiple exchange servers. You could also set an internet facing receive connector to ensure email from (lets say) your banks mail server IP addresses has encryption enforced.
(* - Stops spammer poking at your server to try to find a login that allows email relaying)
(** - just setting this on the connector is not enough by itself to enable anonymous email handling- but the connector setting is part of the equation)