The mistake this prevents
Changing the seed and the prompt at the same time is the most common error in image prompt work. The output is different, and you have no way to know which change caused it.
Holding the seed constant while sweeping one variable makes the difference attributable. It is the same discipline as any other experiment.
Why several seeds per value
A single seed can flatter or ruin a value by chance. Three seeds per value gives you enough to see whether a difference is consistent or an artefact of one particular noise pattern.
Run the same seed numbers for every value so the comparison is like for like across the whole sweep.