Do you often  question why  should you use flower food? 

We have always used flower food as it was how we were trained, but when I asked what it actually does no-one could tell me the exact reason WHY I'm using it,  other than it keeps the flowers alive longer. 

So a little bit of research and this is my understanding WHY we use Flower Food

Stems of flowers are like straws and this is how they drink water, that travels to the head to hydrate the flowers and keep them fresh for longer.

In order for the water to travel freely up the straw / stem it must be clear, free of blockages like gunk and bacteria. Just like when your drinking a milkshake or smoothie, if the straw is clogged you will get no drink 

Water is a perfect environment for growing bacteria as it is alkaline with high PH levels.  Bacteria will clog your straws amongst other things. 

Flower food reduces the alkalinity to 3.5 - 5 PH level, making the water in-fact acidic, bacteria does not like acidic environment so will not flourish in this environment.  No bacteria means clean straw

Try it for yourself,  put the same flowers in two vases, one with flower food and one without and see how the water stays clearer and the flowers last longer when flower food is added.