The brace supports a large part of the roof's weight. Together with the wall post (pillar) and the rafter leg, it forms a truss that is incredibly strong. I have a similar construction in my barn. The wall bulges out in one spot so that almost the entire sill there lies outside the foundation wall/foundation stones. I thought of pulling the wall back into place using a winch and wire. There wasn't a chance. Probably my braces were added after the wall had bulged out. Since I have no desire to dismantle the barn and as it has stood for over 100 years, it is safe to say that it is stable, and the wall will have to continue bulging out. I chose to secure it from further bulging by digging a hole and casting a post as close to the wall from the outside as possible. Probably completely unnecessary, but now I sleep well even during the worst autumn storms.
Braces are present in barns to create interior height. If you tie together the lower ends of the rafter legs with a long board or with wire so they can't slide apart, you should be able to temporarily remove the brace. This is, however, purely theoretical. It depends on exactly how your barn is constructed, assembled, and in what condition it is.