It is considered best practise to specify free draining backfill material (eg gravel) to be placed immediately behind a retaining wall. Supposedly, this alleviates hydrostatic pressure on the wall, allowing more economical designs to be achieved. But the free draining material typically only extents 300mm behind the wall. What prevents the soil behind this free draining material to saturate, impose hydrostatic onto the gravel and then transfer the hydrostatic pressure onto the wall?
Just a few notes here:
With or without gravelly backfill, the retaining wall must be designed for the maximum groundwater anticipated for the site. Your geotechnical engineer shall provide this information.
The 300mm clean free draining material is the minimum requirement, it is recommended to backfill the entire cut (excavation) with well graded gravelly material, either from offsite source, or the selected and processed onsite soil.
Always provide drain pipes and weep holes to facilitate the draining of ground water.
Never compromise the safety factor for the design of earth retaining structures.
I'm not a geotechnical engineer, but I believe the gravel is there precisely to prevent the soil behind it from getting saturated.
That is, if the soil gets saturated, the hydrostatic pressure will squeeze the water out into the gravel, which then drains the water away, thereby reducing or altogether eliminating the pressure on the wall.