New Jersey is an at will employment state, which typically means that an employee can be fired at any time, for any reason that is not illegal. Therefore, most wrongful termination suits revolve around people being fired for illegal reasons, like their race, religion, or gender.
However, it's important to note that a company policy also plays a huge role. If it is breached, you could still have a wrongful termination claim, even if the reason for your firing was not illegal on its own.
For example, perhaps your company has a practice of firing and replacing the least productive worker every quarter. However, you are supposed to be given a warning the first time you hit this level, as a way to encourage you to work harder and be more productive. If you are then the least productive worker a second time, you can be fired.
With a policy like this, if you're fired the first time you hit that mark, with no warning, it may be illegal. You were guaranteed that second chance by the company policy, which you were informed of when you took the job -- and which may be in your employment contract. The company has to honor that policy.
In general, any breach of the conditions of your contract can be grounds for a wrongful termination lawsuit, which is why it's so important to fully understand that contract. This is just one example of how it may protect you. If you are fired illegally, in violation of a company policy or your contract, you may be able to seek compensation or fight to get your job back.
Source: The Balance, "What is Wrongful Termination?," Alison Doyle, accessed Sep. 22, 2016