When you paint a test patch directly on the wall, you only see how the colour looks in that one spot, with that particular angle of light. But light changes across the room – near windows, in corners, behind furniture.
If you paint a piece of card or board, you can move it around: hold it next to curtains, place it behind the sofa, stand it in a darker corner, check it under ceiling lights. This gives you a better sense of how the colour behaves in all parts of the room.
You might discover that a shade you love near the window looks dull on the opposite wall, or that another option holds up better in different conditions.
It’s a simple trick that makes your testing more realistic and avoids disappointment after committing.