As @dylan pointed out, for incompressible single phase simulations we are interested only in pressure differences- the reference is arbitrary. On top of that, the incompressible solvers of OpenFOAM use pressure / density as a field for N-S equations. You can note that if the unit of pressure is m2/s = Pa / (kg/m3).
The reference pressure that you can find in the “numerics” section is required for systems that do not have a fixed pressure value defined at any boundary- for example a closed cavity. Since we deal with pressure differences, we need a reference point. It will be selected at chosen control volume with a prescribed reference value. I personally like to change the “reference pressure” for all my simulations, regardless if they have fixed pressure at boundary or not.
Hope this helps a bit