Summary indexing allows you to follow more of a schema on write system, where you can format the results and aggregate them into kv pairs inside of a summary index. You can also incrementally add to a summary index and include this in your running totals.
You can set the retention on summary indexes for a long period of time (years). Whereas cached search results have a specific TTL which expire in a relatively short period of time. So in short, if you have results which are needed for a limited amount of time, use cached search results. If you want to incrementally add results or query against aggregated results over a long period of time, use a summary index