Everyone can press a button but not everyone is comfortable to inflate a buoyancy compensator orally in the water. It is a good skill to have and it cannot hurt to really master it. It is taught in all entry level diving course as a mean to attain positive buoyancy in the case of an emergency. It is also part of a proper buddy check.
Before the appearance of the power inflator or low-pressure inflator BCs were always orally inflated. Having an on-demand inflator has just made it easier.
Be it on the boat at buddy check, just after the entry to make yourself float, underwater to adjust buoyancy or back to the surface after the dive, there are many occasions divers can practice.
Just make sure you are comfortable and that the conditions are good before practicing. Do not hesitate to use the button in doubt or if you feel you are not positively buoyant enough.
Another reason outside safety is to save a little bit of air every time. Not much but if added to all the other little things can make a difference.