Yeah the primarchs do it, and it's kinda weird. But they are inhuman monsters that have to serve as generals and politicians, so I forgive them.

When space Marines do it the it's kinda like how priests refer to each other as brother, but also battle brother is a rank.

I think it is also there to make things seem more theatric/dramatic. Like just how many books start with a dramatis personae? We're not reading about people, we're reading about characters in a space opera. No matter how you look at it, it's still pretty silly.