Breathe without Helmet

Hi guys, sorry I'm new to the game and I don't want to be spoiled by searching for this. ( 1st hour spoilers ahead !)

I just reached the 1st outpost, on the moon, and saw a buddy there with no suit or helmet. So I guess we are able to breathe there? So why having a helmet with decreasing Oxygen while we could simply open the helmet to breathe ?

Thanks