Corners, edges and areas near ceilings or windows catch light differently from big flat sections. Shadows, reflected colours from floors and furniture, and window light angles can all slightly shift how a paint tone looks.
If you only test a sample in one central patch, you may be surprised later that in corners it appears darker, or near a window it looks more intense. Painting a few small patches in different spots—near a corner, by a window, behind where a sofa will go—gives a more honest preview.
Paint is one of the cheapest changes in theory, but still a pain to redo immediately. Better to experiment on a few square inches than repaint four whole walls.