In an ideal world, patrol cars would have two officers in them. However, in this world of limited budgets for police forces, it is often the case that one-officer patrols are inevitable.
In terms of policing, two-officer patrols are superior. If there are two officers in a car, they can see many more things than if there is only one officer. A single officer has to concentrate on driving and will therefore miss many things. With another officer in the car, this is less of a problem. If there are two officers in the car, there is greater safety for the officers and more ability to apprehend suspects. If a single officer pulls up to a crime scene, more suspects can escape than if there are two. Two officers can also back one another up when entering a dangerous situation.
However, in the budget situation that most cities face, two-officer patrols are much harder to implement. This means that police departments end up with one-officer patrols. In this environment, the good point of one-officer patrols is that they allow a department with limited funds to have at least some police presence in more parts of a city at more times.