Modular gear is easy to expand and complement. You no longer need to spend money on new gear, if during the first trip the strap of shoulder-worn bag chaffed the arm and neck-hanged camera bruised the chest — you can get a shoulder pad for the strap and a camera case. Non-modular gear would have to  be sold or put away instead. Maybe you want to carry everything at your waist belt? Such switch is also possible, with each piece of gear having versatile ways  of attaching. Also, you can exchange individual pieces of gear to mix and match as needed, replacing one specialized pouch with another.