Every ticket has a bunch of properties associated with it, like the agent who is dealing with the ticket, its priority, its status etc. Some of these ticket properties are essential for the running of the helpdesk, like status, priority and the agent assigned to the ticket. Other properties, like source of the ticket, are mainly used to make the filtering of the ticket and applying automation rules easier.
Tickets can be broken down based on ticket properties in reports. This makes it easier to find out things like kind of tickets that come in large numbers or the property of tickets that takes too long to solve, and more.
Companies can create custom properties based on the industry they are in, their size etc. For example, product companies can create a custom property to categorize the tickets into bugs, feature requests and how-tos etc.