When I did a search on 'Why' we should show appreciation to our clients and customers, I didn't find much. I found more on the 'How.' I believe our 'Why' has to be more than to increase our bottom line. If the perception you want associated with your business is that of friendliness, gratitude, and humbleness, then this is an area to focus some time and resources.
Probably showing appreciation to clients and customers may seen a little time consuming. Good habits take time and showing gratitude is no different.
I think this quote sums it up - “Gratitude makes sense of our past, brings peace for today, and creates a vision for tomorrow” - Melody Beattie
*Excerpts from US News article - 'How to show appreciation to colleagues and clients.'