What lights are lit for Parking lights?

[Espeefan]

Kind of a stupid question, but why would you want the cab clearance lights on a separate switch? I've heard stories of truck drivers running out west, late in the night, breaking the speed limit, with all their lights turned off, except for the headlights. I knew an ex-mechanic who said he had an owner operator ask him to wire his lights up like that. Everything else was dark. Just enough to see the road. Nothing more.