As part of your Support agreement with you customers you will have service level agreements in terms of the expected time for a reply to Support Ticket and time for a resolution to a Support Ticket.
It's important to define these for your Support Tickets so you can track how well you are meeting these SLA's and also ensure your Support Agents are working on the highest priority Support Tickets first.
Once defined, your different SLA's for your Support Tickets can be applied through Workflows.