Creating your own ticket type allows you to manage the different kinds of tickets your helpdesk is used for. You may create a ticket type for support requests, one for sales enquiries and another for internal staff HR requests or any other type of incoming issue/request that your business needs to track and manage.
For each Ticket Type you can create a different form depending on what information needs to be gathered in order to support the request. Support Requests may need to ask the user for the Urgency or Impact, but a Sales Requests might not. An HR request might need a dropdown to select a date range or a free text field to collect a body of information. Or you may need to capture a weblink for a technical support issue and therefore would need a custom field for the form/ticket type.
On the main Ticket Types listing page you'll see a list of all available Ticket Types for your system. Everyone starts with a single default Ticket Type, and you can add more as required. See Managing Ticket Types for more detail.