The reason you would need is is because you have an academic paper to write. (You use it in your second class too). You can get away with not having the manual, because you can find many online resources. Here are just a few.
http://www.bgsu.edu/colleges/library...HTU/apa5th.PDF http://owl.english.purdue.edu/owl/resource/560/01/ http://www.calstatela.edu/library/guides/3apa.pdf
The theory teacher I had was very picky about APA, so make sure you do a good job with your formatting, citations, and references. When in doubt, cite! Also, use the writing center to go over your paper. It will make a big difference.
Good Luck!