A loop plays, then again louder or quieter, and you estimate the change in decibels. Balancing a mix is mostly about small level moves, and knowing what 1 dB or 3 dB actually sounds like makes every fader decision faster.
Level 1 changes the level by 5 to 9 dB, with about 3 dB of margin. Every level makes the change smaller and the margin tighter, down to between 0.08 and 0.35 dB at level 10, which is close to the limit of what most people can hear reliably.
Under good conditions, around half a decibel to one decibel on music for most listeners, and less with training and careful comparison. Anything smaller tends to be felt as 'slightly better' rather than 'louder'.
Because louder tends to sound better, and small level differences quietly bias every comparison you make. Knowing what 1 dB feels like helps you catch that bias.