In the common law system, legal principles used to resolve disputes in previously decided cases are used as precedent to decide cases with similar legal issues in the future.
The purpose of case law research is to find relevant precedent. You present the court with case law that is favorable (hopefully) to your side and explain how it applies to your client's situation. (Of course, your opposition is doing the same thing....)
Several options are open to a court when deciding whether to treat a case as precedent to decide a legal issue.
Be careful - not all cases available on Westlaw/Lexis/online are authorized by the court to be precedent. Unpublished decisions are not precedent even if you can find them on Wexisberg.