Breathalyzers measure the blood alcohol content of a person's breath sample that is introduced to it. This BAC level is then compared to that of the state set as the legal limit. If the BAC level of the person is found to be higher than that of the legal limit, they will be arrested and charged with drunk driving.
This exists in every state, however, the actual BAC limits differed among states, as did/do the penalties. For the most part, states have joined together to keep a set BAC level of 0.08. This is to prevent people from being able to drink more before they drive in one state over anohter, which could result in disaster. For example when the limit of one state was generally higher than that of its neighboring state, perhaps an out of state visitor would think he can still drink the same and either end up getting in an accident or charged with a DUI if pulled over.
Aside from the state BAC limit, each state also has a "Zero Tolerance BAC Level". This level pertains to those people in suspicion of DUI that are below the drinking age. Underage drinking is no stranger to society, so underage drinking and driving comes along as no surprise. These BAC levels provide officers with a reference on how to deal with these underage drinkers that have chosen to break another law. There is also a variation in Zero Tolerance BAC levels among states just like regular BAC levels. They can vary from 0.00 to 0.02, with 0.02 being the most commonly used one.
These measurements of BAC levels were last updated on August of 2009 so are fairly up to date, however certain laws may have been passed or changed to alter these levels as well as other aspects. Although the states now uniformly follow the 0.08 BAC level as a standard, there are other circumstances that can affect their penalties. Individual states can have additional penalties and fines depending on the BAC level you have when arrested. For instance, if you have double the legal limit you can serve a much stricter sentence in county jail, or pay double the fine, suffer longer license suspension, etc…
This goes to make an example of those who are driving while belligerently drunk as opposed to those who may have misjudged and had an extra drink at a bar. It can help teach those offenders to be even more careful next time, after being handed such a stiff punishment.
Breathalyzer tests provide the proper, accurate, non invasive method for police officers to be able to check the drivers for their BAC level. With state limits being mainly universal now, it is easier for people to be able to monitor their BAC levels even when traveling through or to a different state. This can prevent from any misinformation leading to a DUI. It also portrays each state as being stronger and less lenient to drunk driving, which is a huge issue in the United States as a whole.