The human voice is effectively a mechanical system designed to create sound. It can be defined in the simplest terms as:
Energy + Vibration + Resonating Chamber = Sound
For a stringed instrument, the energy is in the hand or other instrument that moves the strings to vibrate which resonates off of the body of the instrument. For a woodwind instrument, the human breath causes the reed to vibrate which is channeled into the body of the horn. Like a woodwind player, singers use their breath to regulate the air pressure in the larynx or vocal folds as the sound resonates against the pharynx or back of the throat and mouth. Yes, there's a bit more to it than that but this is the basic, mechanical foundation.
So with the breath as the "energizer" which brings the sound into motion, often the first step in voice lessons is to clearly identify how to use the breath efficiently and then train to use the breath athletically. This is all part of vocal technique building. Breathing for singing involves breathing deeply, which activates the diaphragm, expanding the lower ribs and learning how to regulate the breath pressure needed for the vocal & musical task at hand. Starting off with hissing patterns (or ZZZ's or other fricatives) helps to train the feeling of keeping pressure in the expanded ribs while breathing or singing out on the exhale. And always remember, as in all things, singing practice makes perfect!