There were two reasons for this, historically.
i) The old STBY ATT IND (mechanical) backlight was not available after a failure. In the days prior to ISIS, the dome LT in DIM was a requirement to assure readout.
ii) - not entirely sure on this one - before an essential modification, certain cascading failure could leave the cockpit completely black save for 1 single announciator on OVHD ELEC (bus tie). Pressing this very pb would rectify the situation to a workable state, yet Airbus mandated DIM because completely dark was too awkward, not to mention being unable to find that saviour button in case its internal source had been fried.
These days, I am not aware of any requirement to enforce it. My best informed guess.