You don't unless you can find a legitimate reason to be there.
The people backstage are working and they have a right to be back there. They also have a right to not be bothered by people that don't belong backstage.
The backstage area is cramped, crowded, and dangerous. It's dark, stuff is piled up everywhere, and people are rushing around because they're busy working. Your presence is more than annoying and unwelcome, you're a danger to yourself and others.