It's been 30+ years since I was in a squadron with pilots who
held/needed green cards, but going from memory, it allowed you to
sign off on your own IFR flight plans for cross-country flights
using the civilian ATC system.

I got out of the Marines in 1954. In the early '50s, the above was
true. Some pilots in my squadron refused to accept a Green Card for
precisely that reason. They didn't want to be put into a situation in
which the Skipper would say, "Come on, we need someone to go, and you've
got a Green Card, you can go even though it's below mins."

vince norris