Ticket are request made by client for maintence purpose. In rpoduction support or Service Request support ticket's are opend by the clinet if they face any problem while using the product. It can have different severity based upon the circumstances. Sev 1's are high prority task which need to be completed within hours generally. Again it depends upon client to clien and circumstances.
It's use is through this client report any error or unasked situation that he faced while using the product.