from web site
Stress testing for a GPU means you are making your graphics card work as hard as it can without any stability issues. This process requires pushing the GPU to its limit and confirming that it can handle high workloads without crashing or getting too hot.
It is not like normal usage or gaming, because it tries to go very far with the GPU and possibly make it crash or get too hot. This helps in checking how stable and good the performance of GPU is when put into severe situations.
Learning how to stress test a GPU is important because of such reasons: