A picture speaks a thousand words but I thought it might help a little to explain how this stack, which is actually a pretty good stack job, is created. Bags are typically sorted by line of travel and each airline has a unique methodology as to how those sortations are defined but all for the same purpose of getting the bag to the destination with the passenger at the same time. Bags only going to the next destination or "local" go in one section while bags connecting on the same airline go in another section and then bags connecting to other airlines are in a third location.
Once all that is figured out in the baggage room just below the ticket counter the bags are brought out to the flight for loading on to the plane. Here there are two general goals in mind: maintaining the sortation system so the flight will be "worked" quickly at the other end and loaded for weight and balance to burn less fuel during the flight itself. Bags are heavy things so the sortation that produces the most amount of bags for any one category almost always end up in the very back of the plane. The cargo compartments there tend to be the largest on board and allow for the best flying profile of the airplane while it is cruising along the skyways.
All of this is figured out ahead of time so once the actual loading begins it is typically a straightforward process: the biggest bags go on the bottom, the medium bags go on the top, the soft sided bags, fold over garment holders, kiddie bags and other light things that can't bear a lot of weight go on the very top. Voila. The good baggage handlers take extreme pride in creating the tightest possible stack with the most amount of bags in the least amount of space. The guy in the plane calls the shots on what order he (or she) wants the bags sent up the belt as the stacking job begins. Since they are the ones doing all the twisting and lifting in order to create this work of art they also control the pace and speed of delivery unless they're a rookie and don't know any better or some hot shot who needs to be tossed a lesson.
It is very much a team effort to get everything on board in the time allotted and in the best way possible. For the widebodies all of the packing is done in the bagroom but the stacking rules are the same. When the "cans" come out to be loaded it becomes a question of manhandling the things in to position, any one of which can easily weigh a ton or more fully loaded. The rollers in the floor pictured here help a lot but you can imagine in either scenario back problems are a major concern for airline and worker alike.
A tight stack means it won't tip over like some decaying wall of bricks. It also means using the very limited amount of space efficiently so as to fit all the bags on to the plane without wasting space - and affecting the balance of the plane - or worse, leaving bags behind. There's also bragging rights as far as who is the best stacker on duty or on the ramp in general with all kinds of fish stories to go with it. As for Bowser, the family pet? They always leave room inside the door with plenty of fresh air circulating just for him.