buoyantPressure Sets fixedGradient pressure based on the atmospheric pressure gradient
due to the nature of the PISO-algorithm used in interFoam, it is necessary to have a very "good" pressure field, as the pressure field is required in the velocity correction step of the scheme in order to reach mass conservation (i.e.div(U)=0).
If you look at the PISO-algorithm you will see that this means if the flux is zero (as it is orthogonal to the wall) it is necessary that the second derivative of pressure (orthogonal to the wall) must be zero, too (in areas where the density is constant).
buoyantPressure fulfills this requirement in areas of constant VOF-function / density. I'm not sure about mass conservation if sloped walls touch areas with VOF-values 0<VOF<1.