They cause learners to make desired connections between stimuli and responses. They cause learners to avoid undesirable responses to stimuli. Continuous reinforcement increases the rate of learning.
Intermittent reinforcement contributes to longer retention of what is learned.
Both positive and negative reinforcement can shape behavior. A lack of any reinforcement can also shape behavior. If people receive no acknowledgement of their behavior, they will likely change that behavior until they receive some kind of reinforcement.