GO allows emails to be linked to virtually all other GO modules like TASKS or TICKETS. This enables to create an immaculate track record of all activities to a certain subject.
However, the linking feature works once similar to a one-way street. For example, you want to link an email to a ticket because its content adds relevant information. When you open the ticket you can directly refer to that email and use standard email functions like "forward" or "answer". However, for such a linked email the GO function "linking" or "save as appointment" or "save as task", etc. are not offered anymore.
To transform this email into, let's say, a task you'd have to search through you email folder manually remembering date and time to finally unearth to original email. If the linked email would offer the same feature set as the original email that time-consuming search would not be necessary.