When you create an activity on a company that has a hierarchy parent set, the activity shows on the timeline of the parent records, HOWEVER, without opening the detail of that activity, you can't tell that the task is regarding another company.

It would be useful if the account (or opportunity/case etc) that the activity is regarding shows in the timeline if it is NOT regarding the the record you are currently looking at. (You don't need to see what it is regarding if it is regarding the record you are looking at - this can be inferred by it's lack of a "Regarding" value)