Why is liquid nitrogen stored at low pressure in dewars?

shuttle? I've seen the spherical tanks. Is this high pressure 'warm' liquid nitrogen, or do they let it vent? <<
Pressure tanks are far too heavy for the shuttle to store anything but cryogens for breathing. Certainly they have liquid O2 tanks.
The shuttle crewspace uses basically "air" at very close to sea level normal nitrogen/oxygen ratios and pressures. I'm not sure if they store nitrogen at all. If they do, it's not much. Remember, they're on a rebreather system, of course, so they're just using O2 and absorbing CO2 and smelly gasses. The nitrogen isn't used, so not much extra would be needed. Just enough to make up for any vented to space in EVAs and so on, which isn't much. Perhaps an expert can tell us if they bother to bring up any N2 at all, other than what the crew quarters starts with.
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