This is a general-purpose version of the mean-pressure formula that can be applied to either the systemic or pulmonary circulation — a single reference tool covering both MAP and MPAP calculations.
Yes — this page exists as a combined reference since the underlying math is identical; only the input values differ between the systemic and pulmonary circulations.
When you want a single tool that works for either circuit, or you're computing a mean pressure for a vessel that isn't specifically the systemic artery or pulmonary artery.
The cardiac cycle spends more time in diastole than systole, so the time-averaged pressure sits closer to the diastolic value.
Yes — the same weighted formula estimates the mean from any systolic/diastolic pair, though an arterial line also reports a directly measured mean.