Paint colours change dramatically between a shop card and your actual wall. Buying a full can based only on a tiny square or a phone image can easily lead to disappointment – the colour might look darker, lighter, cooler or warmer than you imagined.
Tester pots let you try a few options on the real surface. You can paint squares in different parts of the room and see how they look in daylight, evening light and under your actual bulbs. Sometimes the shade you were sure about looks wrong, and a backup option ends up being perfect.
Spending a little on testers saves you from paying for large quantities of a colour you end up hating – and from the extra labour of repainting.
It’s a small insurance policy against “Oh no, what have we done?” moments.