One chooses to do a BA degree if, in addition to the COSC major, one has an interest in subjects such as philosophy, history, anthropology or languages (to mention a few examples).
Most areas combine well with computer scence, for example:
Philosophy papers complement the 300-level papers on computability (COSC341) and artificial intelligence (COSC343). See in particular PHIL 222 (Introduction to Formal Logic) and PHIL 312 (Advanced Formal Logic).
Linguistics complement artificial intelligence (COSC343).