Many of us do it. We swatch paints when we get a new color, when we’re bored, or when we’re curious about how they mix. However, we may need the occasional reminder that swatching is really a study into the relationship between water, pigment, surface, and brush/applicator.

Each of those four players contributes to performance; changing one can shift the energy of the overall interaction. All of the paints, all of the papers, all of the brushes bring unique characteristics to the table. Heck, there may be a perceivable difference between the use of distilled water versus tap water. We swatch to observe and to notice these behaviors and relationships.
This data informs our painting or our approaches to painting. Is it required? No. Is it helpful? Perhaps.
It’s like Sociology 101: Watercolor Edition.
So, test the materials that you intend to use for your next grand masterpiece beforehand… unless spontaneity and the unknown are your jam. In that case, have at it!