Using credible sources proves that you now what you're talking about. If you don't have a credible source, you may be spreading false information. This is especially important in healthcare, as people's lives are at stake. For example, if a student studying health tells someone to use a type of drug on a person in danger (passed out/heart attack/etc) because they learned it from a non-credible source, that person's symptoms could worsen or even kill them. Basically, having a credible source prevents the spread of misinformation.