Answer:
Your internal QA department has done all it can to ensure that your software functions as designed. The light is shining at the end of the development tunnel; your product is nearing completion, and the release deadline is rapidly approaching. You know that the application functions on all of your in-house computers. What about the myriad of computer systems of your target users? Nothing upsets your customers more than breaking open that shrink-wrap (or downloading the software from your site) only to find that a conflict exists with the software or hardware they have installed on their computer, rendering your product inoperable. Compatibility testing can help you avoid that dangerous and expensive pitfall