Legal Question in Civil Litigation in California

Sometimes. It depends on what you are trying to prove. They are usually necessary in malpractice cases, for example. (Malpractice is typically a form of negligence, though it can be intentional.)

If a jury can make an intelligent decision without having expert evidence presented to it, then you do not need an expert to testify. No expert is required to show property damage in a car accident, but you need an expert if you are trying to show that future medical treatment is needed.

You are going to need an expert in any case that you are trying to prove the negligence of a professional, such as an attorney (legal malpractice) or a doctor (medical malpractice.)

likely not for the breach of contract cause of action. However, you may need one for the negligence cause of action to explain to the jury why the defendant's conduct fell below the requisite standard of care.