Ortlieb do roll tops to keep stuff dry, everyone else does em to look cool.
I have 2 Ortlieb roll tops, one is for my climbing gear and has chalk powder inside it ( chalk bag doesn't close 100%) and have used that in major rain storms and the powder has still been dry, the other is for stuff I done want covered in chalk. both are allot comfier than my Bagjack of the same volume and I can carry a lot more because of it
I have 2 Ortlieb roll tops, one is for my climbing gear and has chalk powder inside it ( chalk bag doesn't close 100%) and have used that in major rain storms and the powder has still been dry, the other is for stuff I done want covered in chalk. both are allot comfier than my Bagjack of the same volume and I can carry a lot more because of it