Light direction and quality change from room to room. A colour that looks warm and soft in a sunny living room might look dull or too dark in a north-facing bedroom or narrow corridor.
By painting test squares in at least two or three locations – bright room, dimmer room, maybe a hallway – you see the full personality of the shade across your home. A colour that works acceptably everywhere is usually a safer choice than one that looks amazing in only one spot and odd everywhere else.