The way I see it allocation is for resource planning indicating demand which your supply id capacity is to match.
ETC is for project planning and timeentry for project tracking. In real life you also use the projects those resources you have in planning. Therefore the smoother operation you have if your resource and project planning are in sync.
Those two jobs help you in that. Estimate from allocations takes the allocations and sets the effort (ETC) equal. If I recall correctly the down side is that the system does not know how you want to distribute the effort between your tasks therefore all the effort is put on the effort task only. The allocate from estimates does the opposite. It sets the allocations equal to the effort estimates of the resources in all the tasks to make the resource planning demand more accurate.
The allocation start and end dates also serve to help to have the resource planning more accurate. By default they are the same as the project dates, but if you know better the allocation can be for less time. If you want them to be outside the project dates you would have to reset the project dates according to your need. However, usually the project and task dates determine when you need resource and not vice versa.