It's common to hear of students who come to college without knowing that there are multiple formatting styles; “I only used MLA in high school” is a refrain we in the Undergraduate Writing Center hear all too often. MLA has its uses and is fantastic for some disciplines, but other disciplines, such as history, emphasize different things and need other formatting styles.
Turabian style is an adaptation of the format used at the University of Chicago. The style was originally compiled into a small pamphlet by Kate Turabian, a graduate school dissertation secretary for almost 30 years. Turabian was the standard-bearer for high academic achievement at the university, and her small pamphlet has since been expanded into A Manual for Writers.
Originally meant for the high standards of graduate dissertations, Turabian formatting style has become most associated with history for its value of credibility and accuracy over the recency of information. In contrast, APA format places emphasis on the date of publication, because more current studies tend to be valued over less current studies.
Turabian values the credibility of a source; the distinctive use of footnotes reflects this value. Readers can come to a claim or a fact and immediately look to the citation at the bottom of the page to judge the credibility of the source. Flipping back to a bibliography or reference page would slow down that judgment and could potentially hurt the construction of the argument. Historical and religious disciplines need this easy way to display the source of quotations.
Now when your professor places a high emphasis on using correct format, you can understand why!