Simulating many simultaneous users to find out how a system behaves under expected and peak traffic: where response times degrade, what breaks first, and at what point it stops recovering.
Capacity is not something you can reason about reliably. Launches, sales, and campaigns are exactly when a product must not fall over, and that is also when nobody has time to investigate.
We load-test before campaigns and major launches against realistic scenarios rather than a single endpoint, fix the bottleneck that actually appears, and document the limits so you know what the system can take.