Simply put, bibliographic managers are designed to help you keep track of sources you find during the course of your research. Rather than relying on web links (which may not be permanent), or trying to track down articles you found previously, you can use a bibliographic manager to add articles as your research progresses. In most cases, you can also create a Works Cited or References page directly from the manager.
As is the case with many citations you find in databases, the citations produced by bibliographic managers may often have errors. It's your responsibility to double-check any citations for errors. Be sure to check any citation you pull from a database to verify it's correct. You can use the MLA Handbook, the APA Manual or the Purdue OWL MLA or APA guides.