To conclude the GO Fest in Chicago, Pokémon GO once again surprised everyone. They allowed raids to appear that were meant for all players on-site. It was a huge stress test that presumably went well.
This test is about: Niantic called all players from the GO Fest on Sunday to a big group photo. There, a Niantic employee took the microphone and asked if the players wanted to participate in a test.
Suddenly, numerous gyms appeared around the stage from which a raid hatched. Niantic wanted to test whether the servers could handle so many players simultaneously in a raid.

Stress Test at GO Fest – What’s Behind It?
How did the test go? After a few minutes, a Meowth hatched at each gym. Players could enter the lobby with a free raid pass and defeat the Pokémon as usual.
In the end, very few players had real problems during the raid. Even though thousands of trainers participated, the servers held up, and the stress test went well.
Why do this? An author from Pokémon GO Hub was present and asked the Niantic staff exactly that. They only confirmed that they wanted to see how thousands of players could complete raids in such a small space.
Now one can only speculate why they want to know this. It is also suspected that at the next GO Fests, raids will already be possible. In Chicago, this was not possible on the grounds. There were no gyms. They likely did not want to overload the servers too much.
Since the tests were successful, one can already hope that one can battle in raids in Dortmund. There, Niantic might come up with something special for the GO Fest.
What do you think about the stress test? What could be behind it?