One reason I and a number of people do that: to cover the possibility you might be at the wrong door/house or ringing the wrong doorbell.

I had both of those problems at my previous residence. It was an apartment complex with some N½ addresses. Visitors would often miss that bit and be at the wrong door. On top of that, the doorbells were counterintuitive: they were arranged vertically, but the bottom doorbell would ring the top apartment and vice versa, so I forever had visitors and deliveries ringing the wrong bell (and actually mostly for the other units ringing mine.)