Fast charging is a very useful feature that many consumers consider "a must have" when deciding which handset to buy. On the other hand, even if using this with today's batteries should not lead to a shorter life span for them, disabling fast charging should be available for those who might need it.
Last year, Samsung decided that removing the ability to disable fast charging from TouchWiz is a good idea. Now they bring it back with the Android 6.0 Marshmallow-based updates. To turn off fast charging, one simply needs to go to the Settings>Battery menu and find the Fast charger section with the Fast cable charging toggle inside.
One of the main reasons for disabling this option is that most handsets get pretty hot when being charged with it enabled and some users want to use their phones while charging them. Even more, since many smartphone users charge their handsets overnight, it does not matter how fast is the process, since any smartphone battery used by Samsung handsets at this time can easily get a full charge in a few hours.