Why are 'step climbs' used on long distance flights ?

Question

Question ATPL - Airline Transport Pilot license - Performance

QuizVDS · Accepted Answer

To fly as close as possible to the optimum altitude as aeroplane mass reduces.

QuizVDS · Answer

To respect ATC flight level constraints.

QuizVDS · Answer

Step climbs do not have any special purpose for jet aeroplanes

QuizVDS · Answer

They are used for piston engine aeroplanes only.

Id 90 - ATPL - Why are 'step climbs' used on long distance flights ?