It’s used because it’s a consistent workload to ensure a fair comparison, and long enough to make sure the performance seen is not just burst. Someone who plays games, edits smaller videos or photos all day, uses heavy web apps, compiles code, etc., can apply the result of a large video render to their purchase decision even though their work doesn’t aggressively use the battery as fast as possible.

Hopefully that helps. In your original example, you cited someone editing FHD doing fine with a five year old laptop, and now we’ve talked about why larger formats are used, and why someone upgrading a laptop would look at a benchmark of an intensive process, even if they themself don’t plan to run that specific process.