I am referring to entry or passage doors with automatic closers, and here are some reasons as below.

Firstly, a little background information. Most door closers are hydraulically controlled, meaning they are filled with oil and use a piston and a series of staged adjustable valves in concert with a strong spring to control the closing action of the door. Most closers have at least two stages, a closing stage and a latch stage, with more expensive ones having four or more, such as a back check (a kind of hydraulic door stop), or a delay stage that keeps the door open for a specified time before letting it start to close. There is also a spring force adjustment that can be adjusted for different width doors, as a wider door will need more spring force due to mechanical advantage, think of it like a small pry bar and a large pry bar, the large pry bar can hold more weight with the same amount of force applied to the handle, a large door will resist being closed more than a narrow door.

All of these adjustments are designed to work together to give you the desired action. Generally, unless you want a delay action for wheelchair users, janitorial closets, etc. you would set the door to fully close in about 5-7 seconds. This makes the door move relatively slowly, and if it has a stiff latch bolt, such as on some panic bars, or smoke/draft seal around the perimeter of the frame that has to be squished, that speed will not provide enough inertia to actuate the latch. Now, you could increase the spring strength to push harder and actuate the latch, but go too much and the door will be hard to push open. So, the latch valve is used to speed up the last few inches of the closing. If you had a set of French doors or a bathroom door with only pull handles and no latch, you would do the opposite and slow down the latch speed even more than the closing speed to make it nice and quiet when it hits the frame.

The other problem that can arise, especially in larger buildings, is the air pressure created by the heating and air conditioning system. After the air system has been balanced, many doors have to be adjusted to close harder and faster to overcome the pillow of air pressure. This can make them slam when the air is off, and unfortunately sometimes you cannot fix that issue without redesigning the building ventilation system.

Now, how to adjust the closers? If you think that the latch speed could be slowed and still reliably latch the door, you can adjust the set screws. One will be labeled L for latch or it may be the screw closest to the end of the closer if the screws are on the side of the body. Sometimes there will be a sticker to show you which screw controls which stage. However, a word of caution is in order here. Commercial door hardware installers and the manufacturers of that hardware carry a large insurance policy to protect against any door becoming a life hazard if it were to malfunction. If the door in question is a fire rated door, that closer is there as part of the fire rating function. If you were to soften the closing and cause the door to latch unreliably as result, you could incur liability in case of injury and death. If you are not sure, ask a professional.