During my time at SRC, the question of how to combine multiple (10+) spatial objects into 1 object has been posed several times. Everyone's initial instinct is use the Spatial Process tool contained in the Spatial tool pallet. At first glance of the Spatial Process configuration window, one would logically think that this would be the best place to combine multiple objects due to the actions listed. The only problem is that the Spatial Process tool is only meant to combine 2 spatial objects into 1 at a single record level.
When the ultimate objective is to spatially aggregate data, the Summarize tool is advised. In the past, I always thought of data summarization as a means for statistical analysis. For example I have 5 numbers and I want to know the min\max or median for the stream of data. I never really thought about data aggregation in spatial sense. Alteryx, however, combines the traditional data summarization methods with spatial summarization methods to enable construct detailed polygons from many smaller parts.
Now when you need to aggregate smaller geographies such as zip codes and combine them into 1 trade area, use the Summarize tool.
