Contacts are divided into Clients and Agents; Leads are the subset of Clients you are actively working with.
Because most of your day-to-day interactions will happen with your Clients, Events and Tasks can be linked to a Client to help you keep track of all your interaction with that Client.
Besides having a handy list of Agents and their contact information, Agents can also be linked to Listing Showings and Buyer Showings so you can easily track your interaction with other Agents. Running late to a showing and want to notify the agent? Simply bring up the Event details to view the Agent's contact information.