A sound plays with a certain stereo width and you place it between mono and fully wide. Width is one of the most overused effects in modern production, and hearing it accurately is what keeps a mix wide without falling apart in mono.
At level 1 your answer can be up to 28% away and still count. The margin tightens every level, down to 3% at level 10, where you're telling apart images that differ only slightly in spread.
Differences between the left and right channels. The more the two sides differ in timing or tuning, the wider the image feels; when they're identical, the sound collapses to the center.
Because many playback systems (phones, some speakers, clubs) sum to mono, and some widening tricks partly cancel out when that happens. Hearing width well helps you know how much you can safely use.