Same seed, same prompt, same settings, same frame. That reproducibility is what turns generation from a slot machine into an experiment: change exactly one variable and you can see what it did.
It is also the cheapest debugging tool in the stack. When a set stops working, holding the seed and varying one term at a time finds the culprit in minutes rather than in a hundred rerolls.
QUESTIONS PEOPLE ASK
What does the seed do in image generation?
It sets the initial noise. Fixing it makes results repeatable, so you can isolate the effect of a prompt change, a guidance change or a model change.