Many people in business, customer service or other people-facing roles do not know how to say yes. I don't mean to accept something, I literally mean that they seemingly do not know the word "yes." They say "yip", "yup", "yeah", "uhuh", "okay" and a few other similar responses, but for some reason they never learned, or more likely (all kidding aside) they didn't develop the habit of saying the proper word "yes" when doing business.
What's the difference?
They both get the same point across but the tone and overall feeling is different. "Yes" is a strong, confident, professional answer that is clear, concise and reassuring (to the customer/client). The other casual choices are the opposite - weak, unclear, unprofessional and often give a feeling of apathy. This feeling reflects not only on the employee but on the brand of the company.
When I train customer service teams or front-line workers, I always make a point to mention this point. Learn to just say yes. Calmly, quickly, and positively, give the other person a sense of confidence, relief and trust. Give them a "yes!"