Whether you use a reference manager or keep track of your references manually is eventually a question of personal taste.
For researchers, a reference manager can save valuable time. For example when the journal you intend to publish in has certain requirements regarding citation styles and bibliography layout. Endnote contains more than 6,000 styles that automatically generate the layout and information required by the journal.
As a general rule, you could say that the bigger the project and the more references were being used, the more reasons you have for choosing a reference manager.
Their primary advantage is they enable you to build your own library containing all the sources you use during your project. You can add your own metadata, save pdf files, make annotations and much more.
When you write a book, book chapter or journal article, the publisher often has requirements regarding the use of certain citation styles and bibliography formatting.
At Endnote.com you can download more than 6,000 different output styles from a variety of publishers from all over the world and import them into Endnote and Word.
The Royal Danish Academy does not require researchers to follow one certain citation style. Therefore, in assignments, dissertations and publications, the authors themselves decide which style to use. The most common styles in use are APA 6th., Harvard and Chicago.