It's meant to create an empathetic relationship between the staff member and the customer, in case you didn't know. The 21-year-old customer relations consultant asks how your day has been or what you're planning for dinner and as you answer a warm, snugly blanket of empathy envelops you both. Naturally you'll want to return for another coffee or sandwich or beer as quickly as possible to be re-enveloped in the empathy. That's the theory of empathy training, and that's why, suddenly, every retailing junior wants to know about your weekend.
There's more science in it, too, than the previous Americanism that spread like a plague through retail workers a decade ago. That was "have a nice day", and while it made Australian customers wince for the first two or three years it is more common today than hello and goodbye. Try substituting it with "missing you already!".
At least, though, the instruction to have a nice day requires no response, and that won't do for a relationship built on empathy. The customer needs to respond, and so the empathetic questioning is designed to require a response, and a friendly one at that. For that it relies on an obligation to be civil and, these days, inclusive.
Give me the old please and thank you, even if the time taken to make the coffee or the sandwich is broken by nothing more than silence. Give me "have a nice day" if you wish. But my day, my weekend, my plans for anthing are none of any stranger's business.
Do you, too, resent the commercial expectation that we'll welcome staff members' inquiries about our lives? That we're open to empathy with whoever makes those inquiries? Bring back a little formality, I say.