REI Minimalist Bivy – Raigutter?

Can someone explain the rain gutter to me? I guess I don’t see it or how it’s supposed to help?

BTW I’m new here, been lurking for a while, reading LOTS on tents and shelters and lightweight stuff. a little overwelmed but I’m slowing learning.

MY first thing to do for UL is lose some weight in my belly.

Trying to train back into shape for the inyos.