I don’t know if you have read the help text on the aggregators but it is quite useful to understand. There is a good example too.
The crucial text:
The module from which the bundle aggregation will start. The source module is usually an iterator or a search module that outputs a series of bundles. Once you setup the aggregator’s Source Module (and close the aggregator’s setup), the route between the source module and the aggregator will be wrapped in a grey area to visualize the start and the end of the aggregation.
I think of the aggregators as having an input of zero or more output bundles. Then the aggregator combines the result into one output bundle. If you want to manage the data from multiple output bundles an aggregator is the way to go. Is this what you want to do? If it is not then aggregators are not for you, and some other array/object manipulation may be in order.
I just wrote a response to another topic that involved the use of 3 aggregators in a row in a scenario. Maybe that will help you too.