Related activities are activities that are connected to each other by the author to maintain a thread for an event focusing on one subject. When you relate activities you can display all the notes associated with activities that are related on one screen without viewing any unrelated activities for the same contact record.
An example of a typical related activity would be the action of calling or taking a customer call for any reason and having to make several calls on different days to conclude the original conversation. Let's say you called a customer and they had left for the day. You would record your original attempt to call, make a note that they had left for the day and complete the original call. You would than create a related activity (related to the original activity) so that you could call later.
The value in recording all activities, especially in customer service is that you have a detailed record on whom you called, when, at what time and the result of the call. A customer complaining that you didn't call as promised could easily be assured that you had with dates, times and why you weren't able to get through to them. Perhaps, they were in a meeting or at lunch or otherwise unavailable. Every attempt to contact them should have a related activity back to the original outbound telephone activity. Even if your attempts span different dates you can easily display related activities to see all tries and why they might not have worked.