Because everybody is doing it :
Nothing done : HRtP, MS
Easy Cleared : PoDD, LLS, PoFV, UFO, UNL
Normal Cleared : SoEW, EoSD, PCB, IN, SA, IaMP, SWR
Hard Cleared : MoF
Extra Cleared : EoSD, PCB (both), IN, PoFV (Komachi only)
Current goals : SA Extra, UFO Normal. Maybe try HRtP and MS again.