Linking Tasks allows you to ensure that related tasks are easily available for you to toggle through.
It also allowed you to record if a task is blocked by another so you can ensure the tasks are completed correctly.
Linked Tasks are available for all Task Types on the left hand side of the Task.
Once here you can select Link Task where you'll need to select the linked task and how it is related.
The Tasks can be related in three ways:
Relates to - This will just note the tasks as related
Blocks - This will disallow the linked task to be completed until this task is completed
Is Blocked By - This will disallow this task to be completed until the linked task is completed
You will be able to see the Linked Tasks on each of the Tasks and you're able to link multiple tasks.