It doesn't really. Pools with sand filters have to rely heavily on more chemicals (flocculants, coagulants, higher chlorine levels) to maintaining water clarity. Residential sand filters are crap.
in uk we use magic lube. Vasoline is a No NO , take an o-ring out thats been vasolined and just touching the o-ring will cover your hands in black dissolved rubber that takes 2 days to get off.
Some pool professionals say gravel helps to support the sand and reduce the strain on the lateral assembly. You just have to find the right size gravel. Small enough to fill in the voids around the lateral assembly, but not so small that it is disrupted when backwashing. But no, it's not required.