When you collect SQL workload and define some changes to analyze, the Analyze Impact module will first get plans for the SQL in the workload, then apply the changes and get plan again, and finally undo the changes for you. Analyze Impact will not execute your SQL but it will check if there is any plan change in your SQL. It then uses the Cost, IO Cost, CPU Cost, and Time figures in the original plans and new plans to calculate overall reductions for all SQL in the workload (with the execution number taken into account). So Analyze Impact allows you to quickly find the potentially affected SQL of your changes and let you see how the plans will change in the affected SQL.
On the other hand, Benchmark Factory is more on the benchmark testing side. It collects SQL workload and then replay it in the exact same way or with modifications like multiplying the workload to simulate a scalability test. It also provides industry-standard benchmark testing like TPC-C and TPC-E.
Hope this answer your questions.