Tickets can be related to each other for a variety of different reasons. They might be the same or similar issue, they might belong to the sub set of customer/user or they could be related to the same wider technical issue. For all these reasons, and more, it's useful to be able to relate tickets together allowing your Agents to cross reference and more easily resolve related tickets.
You can link tickets together by using the Related tab on a ticket. There is no limit to the number of tickets you can relate to each other and you can unlink them at any time too.