Poor typeface choices can make a document hard to read or unprofessional to look at.
It’s important to select the right fonts for your publication. A good rule of thumb is to use no more than 3 typefaces in a document. Consistency throughout a document or website is helpful. It keeps the document clean, readable, brings unity and one identity. An article "Do’s and Dont's of typography" by Sam Jones provides an impressive list of basic typography that is beneficial to both graphic and web designers.