In case the Initial Load failed or has to be commenced a second time, the End User simply has to restart it. Just imagine a new fact attribute should get added therefore all have to be reloaded - better think when building the flows rather than having tons of tasks that have to be followed or your load will fail or worse have some data twice then.
Tables already loaded get truncated automatically via either a script or using the "delete data before load" option.
And the PreLoad Stored Procedure takes care about all the Indexes or other database objects that might have been created already and should be disabled first.