It's not just the point of asking if you need help - it's the most basic, time-honored COURTESY OF AT LEAST SAYING HELLO - acknowledging one's presence 1) as a human being and 2) as a customer in THEIR shop or store.
I don't like to be followed around a shop either - I want to look & enjoy ! But if the staff is "too busy" (or indifferent) to acknowledge the customer's presence, then it's time to take our business elsewhere !!