So that a manager can go to one place (the History List) to answer the question, what work did we do today/ yesterday/ last week ?
Currently, there is no way to create a lookup of all the tasks CREATED (not scheduled) on a specific day. (As Roy L has pointed out, you might be able to get this from a report. But that's not a lookup).
So, if a history record was created when a task was created, the History list could tell us what tasks were created and what tasks were completed in a designated time frame.
And, while we are making changes, lets have another history record when a task is rescheduled as well.