Tasks are used to split up the work to solve a ticket. The reason for this could be the complexity of the ticket or the fact that the work should be performed by different people or groups. Split up the work in tasks also gives you possibility to keep track of the work steps and used time for the different stages.
With tasks you can keep track of steps that are needed to solve a ticket. Some types of tickets often contain the same tasks and they need to be solved in a defined order. Then you can define a Task Template to make it simple to add all the tasks needed and get control of the work steps.