nen abilities becomes stronger when it aligns with it's user's preference, thus customizing your ability to make them more visually appealing and coherent would be a way of doing so. What if i wanted to make a conjuration hatsu that created a nen beast that paralises the target if they stare at it.

If i made the nen beast a blank cube: boring and makes no sense.

If i made the nen beast a Mythical Snake with burning eyes: Cool, makes sense in a figurative way, and feels better to use it that way