Back when I was a travel agent, we didn't have computer generated boarding passes... (Yes, that is me, sitting at my desk in 1986. Check out my computer!)
When your company under promises and over delivers, you will create experiences and memorable moments for your clients that will affect their habits for a lifetime.
Do you remember the last time you received a bit more than you expected?
Maybe you were shopping online for your groceries and when you unpacked your delivery you found a couple of free samples or an extra loaf of bread? There's a good chance that, even though these occasions may be rare, when they do happen you don't forget them. You might even tell your family and friends about it.