Bandoleers or sometime spelled bandolier according to most experts came about in the 1600’s because weapons back then required the shooter to essentially put the cartridge together each time by adding their own powder, wad and shot. The materials were carried in little pouches attached to belts slung across their chests. Each pouch carried the shot and wadding needed to fire one round. Of course, as the years went by the cartridge became self-contained and they too could be carried in the bandoleers for easy access. The ammunition is literally at your fingertips when crossed over your chest. This makes combat reloading more efficient.
Typically, when reloading your shotgun it is at waist or even chest high and having your rounds there makes the job more streamlined. The military adapted bandoleers to carry already loaded ammunitions clips to increase the number a soldier could carry. Belt pouches are still used but they have a limited capacity. Shotgun bandoleers can hold up to 50 cartridges or even more in some cases. The cartridges are secured all facing the same direction making loading much easier that grabbing them from a belt bag or pouch.
Some shooters may have receiver mounted and/or stock mounted cartridges carriers and they can be replenished quite easily from the bandoleers. You have eliminated having to reach for a belt pouch because your hands while holding the shotgun are just inches away from the cartridges.
Bandoleers can be made of heavy canvas with stretchable holders or made of heavy-duty nylon and even leather in some cases.
Can be worn across the chest or at the waist and this particular one has carriers for rifle/pistol ammunition as well as for shotgun shells.
One advantage to a cross-chest shotgun bandoleer is it keeps bulky pouches off the waist that can get in the way in tight spaces. When clearing a building or moving through heavy brush you do not want items at your waist catching in small openings or on vegetation. When crouching behind cover a bandoleer gives you much faster access to your shells as well. The biggest advantage particularly in a combat situation is the number of shells you carry on you. You always want to have too many shells, because not having enough is never a good situation. The weight is not significant when carrying ammunition across your chest because your shoulders can take the weight better than your waist and you do not have the sag that some shell holders cause at the waist.
Using quick connects allows you to remove the bandoleers one handed for easy loading of the belt. Once loaded connect the buckles and then place across the chest because it can be awkward trying to connect the buckles after you have it over the shoulders. You should always carry them across the chest so they do not slip off the shoulders.
You may see in the movies where soldiers and others are carrying ammunition belts over one shoulder. What they are carrying is not a bandoleer. Typically, the clips are for a 60 or even a 50-caliber machine gun. The belt clips are fed into the loading port on the sides of the weapons and the metal clips are ejected as the shell enters the loading port. The ammunition belts would not be defined as bandoleers.