Case – lower and uppercase – serves the purpose of helping reading and therefore meaning in graphic texts. Nothing substantially changes to the pronunciation of a word. It is therefore a wholly written feature of language that is not apparent in spoken form.
Concordancing software often allow you to choose between being case-sensitive or not. At times, it may be desirable to make a distinction between uppercase and lowercase in doing corpus linguistic analysis. An example of such desirability may be in the case where a text is abundant with the word token Will, as in the nickname for William, in which case the inflated frequency may be mistaken for the modal auxiliary.