Light behaves differently in corners than it does on wide, flat sections. In corners, shadows and angles can make colours appear deeper or more intense. If you only test paint in the middle of a wall, you might miss how it looks in the areas you actually notice most in daily life.
By painting test patches near corners, around windows and next to trim, you see how the colour interacts with depth, natural light and other surfaces. A shade that feels soft and calm in the centre might feel too dark or muddy where two walls meet.
Corners also show how well the colour works with adjoining rooms, because they’re often where you catch glimpses of another space.
Testing in varied spots gives you a fuller picture, not just an idealised version.