The law being the law, has a very complex set of rules as to what can be admitted in to evidence in court, how, where, for how long, by whom and so on. All lawyers need to be able to access the rules and lawyers working in litigation must know what they are.  Studying the law of evidence is fundamental at law school because unlike what you have been shown on American TV shows, you can't just spring a crucial bit of evidence on someone at trial out of nowhere.