As someone who has worked in Retail, I can tell you that I was taught to use the `10 second rule' - if a customer is on your department, then you are supposed to (At least) speak to them within 10 seconds, even if it's just a "Hello." When I was moved up to the Footwear department I used to greet customers who had just come up the stairs, or got out of the lift, even though I knew that they'd been greeted downstairs. We were also taught to ask if the customer needed any help after we'd greeted them - it was more to let them know that we were there (And that we'd acknowledged their presence in the shop) if they needed any help.
I could tell by looking at someone whether or not they stood a cat-in-Hell's chance of finding what they'd come into the store for. Some people had a look in their eyes of `I'm completely out of my depth in here'.