Reinforcement is something a dog seeks to obtain, something they will work for. This could be a stroke or a piece of food. Reinforcers that animals innately like, they do not have to learn to enjoy, are called primary reinforcers. Reinforcers that dogs are conditioned (learn) to enjoy like toys and balls are called secondary reinforcers.
Dogs are natural predators and are energy efficient. Dogs like most animals do things, exhibit behaviors, to access something they consider of value.
Contrary to popular belief dogs do not exist, and are not just waiting, to please us. When teaching any student, human or animal we first need to understand how to motivate them and make their learning worth their while. Not all behaviors are worth the same reinforcement. Complex behaviors need us to pull out the big guns; the really yummy treats, whereas a simple sit may just warrant a small treat.
Using food in training is very motivating for the dog and is also a powerful reward. During the training process we use food to encourage and reward new behaviors. Food is then very quickly replaced with life rewards, such as petting, toys, access to the outside, dinner, going for a walk.
It is a valid concern when people express strong opinions about not using food in training because they consider it bribery. Incorrectly using food in training can create a dog that will only behave when food is present. The goal is to make sure that food is being used as a reward and not a bribe, there’s a big difference and a professional trainer will understand this.