Picture this, your flying in the evening. You set off with the cloud getting lower, and mist occouring on the ground, bit of rain too. It's getting darker, you're flying over large unpopulated area's with no lights on the ground, and it's just got that bit darker, viz has reduced to 2.5 miles, you can't make out the horizon, the cloud has got lower, the mist thicker, the horizontal viz worse. It's now fully dark, no visible horizon, 1/2 mile viz, cloud at 900', mist on the ground ... basically there's no point looking out the windscreen. I've been there as a pax in a Warrior, P1 was half way through an IMC, and I'm here typing this. Think of it as an insurance policy!