A useful feature of the Activity Timeline within a profile page is to track changes made to fields related to the profile.
If there was a change that relates to an organization or person on a list you'll see that change noted on the respective profile page!
Note: If you do not have access to certain lists, field updates for those lists will not surface for you.
The following field types - when updated - will appear on the Timeline.
For example, if I were to change the Status (ranked dropdown) on a particular organization:
It will show that status change on the Activity Timeline in their respective profile: