A manager who does that has most likely been burned in the past by a rep/reps who weren't out there every day doing what they are supposed to do. That is a manager who is approaching the business from a negative managerial style, meaning they are trying to "catch" their reps not working by surprising them. That is an insecure manager who will make your life a living Hell. Imagine, each ride along some d-bag asking you why you don't have a lunch and learn, or two, booked for the day, why you aren't lugging Starbucks into your top office, why you didn't start at 8 and end at 5, and why you aren't sticking to the 2 week routing that he/she most likely requested months ago. Good managers give you at least a few weeks head's up that they are coming out with you, out of respect for your time taking them around and so that you can prepare your offices who might be sensitive to a manager popping in.