Your helpdesk comes with default triggers which carry out basic helpdesk processes like:

• Sending an email to the user when an agent replies to their ticket
• Sending notification emails to agents based on their notification preferences
• Sending a notification email to the user when an agent creates a new ticket for them
• Carrying out email validation for a new user (if enabled on your helpdesk)

These are explained in the Default triggers section of the admin manual. 

Be careful not to change or disable the default triggers on your helpdesk until you understand what they do.